Goldfish, Carassius auratus auratus, are one of if not the most popular aquarium fish ever. Most aquarists take their first step into fishkeeping with goldfish. Unfortunately most of the time they start with something like a ten-gallon aquarium at best. Many features make these fish a great choice, but there are certain needs that need to be fulfilled. This article will focus on fancy goldfish, the round-bodied varieties.
One of the most important issues, and most often overlooked, is tank size. These fish get much larger than the little one inch babies swimming around a store’s tanks. There is some variation in size based on breed. In general they can reach fist size for their body alone, not including their fins. That is a large fish compared to what most people bring home. They are also schooling fish. They like the company of other goldfish, so a school should be established. A school is usually a minimum of six fish. One of the best guides for determining minimum tank size for goldfish is to start with 20 gallons for one and add an additional 10 gallons for each additional fish. This puts the minimum tank size for a school of six goldfish in the 55-75 gallon range, and bigger is always better.
Another vital aspect of goldfish care is water quality. This is determined by filtration and water change schedule. Filtration keeps the water clean between water changes and collects the debris for you, the aquarist, to remove. Goldfish are not necessarily dirtier than other fish for their size. They are about equal to cichlids weight for weight. But lots of filtration is needed for goldfish. I prefer AquaClear hang-on-back filters. They have great flow, tons of media, you can customize the media, and you can reuse the media over and over. It is important to clean any filter at least monthly. Many people wait until the flow is reduced, and some filters claim they can go months without cleaning, but the debris in the filter will breakdown into ammonia and eventually nitrate, as it rots. This greatly reduces water quality.
In general, whatever a filter claims it can handle should be cut in half. This should be considered the absolute minimum for goldfish. The more filtration there is on the aquarium, the better. There is no such thing as too much filtration. There can be too much flow. Even in heavily filtered aquariums the aquascaping usually reduces flow, creating areas of higher and lower flow. Most goldfish can deal with moderate flow rates, especially when offered lower flow resting areas. There are some breeds and individuals less tolerant of higher flow, so when there is evidence of stress due to too much water flow, the appropriate changes to the aquarium’s setup and filtration should be made.
Water changes are the most important aspect of water quality. Weekly water changes are the best way to keep water quality up. The minimum water change schedule is determined by the nitrate concentration, which should be maintained at no more than 20ppm, the lower the better. This usually ends up being somewhere in the range of 25-50% per weekly water change. Nitrate can reduce growth and stress the fish. There are other chemicals in the water that can have the same affect like growth inhibiting hormones and dissolved organic compounds. In general if the nitrate concentration is within a safe range, the other possibly problematic chemicals will be too. Please also read the Water Changes Article.
I believe any freshwater tank should have a strong air stone, but with goldfish I would consider it absolutely essential. They love the strong aeration. The flow helps keep debris moving until the filters grab it. And some goldfish just plain enjoy playing in the bubbles. Strong air stones will ensure that the temp does not make aeration an issue. Goldfish seem to be more active and have fewer health problems when they have a strong air pump.
They also provide a great backup in case the filter ever stops running (impeller breaks, dies, or becomes clogged, pump dies, etc.). Goldfish love highly oxygenated water and strong air stones are the best way to provide this. For more information on aeration, air pumps, and air stones please read the article on how Air Stones Actively Aerate the Water.
Goldfish are not true coldwater fish. They are eurythermal meaning they can thrive in a wide range of temperatures. Goldfish are very hardy and can tolerate a wide temperature range. The higher the temperature is, the higher their metabolism. This means more waste production and a higher need for oxygen. The warmer the water is the less oxygen it can carry making strong aeration that much more important.
Even tropical temperatures are not a problem for goldfish. When my goldfish bred in the tank it was in warmer temps, about 78F if I remember correctly. The higher temps do boost their metabolism though so if you are not doing large enough water changes this problem will be amplified in warmer temps. So make sure you are doing large weekly water changes and watching the nitrate.
It is important to remember that although their natural range includes cooler areas, it also includes fully tropical areas as well. In addition, fancy goldfish are raised on the exact same farms as all of our tropical fish. So they were born and raised in the tropics of Florida, Southeast Asia, etc. Tropical temps are not only nothing new to them, it is the only thing they know until they get to some wholesalers’ tanks and all retailers’ tanks. In addition, goldfish have been naturalized all over the world, in areas such as Saudi Arabia and Brazil. If these aren’t tropical than nothing is. Anyone claiming they are coldwater fish simply has no clue of the reality of goldfish. So if anything warmer water will allow them to settle in and get comfortable that much sooner.
I keep my goldfish tank at the low end of tropical, about 76F. This allows them to grow well and be happy, while not turning them in to excessive poop factories like they would be at 82F.
There are many options for substrates in the aquarium, but not all are suitable for goldfish. It may seem as little more than a decoration to make the tank look better to us, but the substrate can have a huge impact on water quality, fish health, and their well being.
The most common substrate in freshwater aquariums is pea-sized gravel. This is not a good option for goldfish. For one, as in any tank, any sized gravel can trap a lot of debris which can cause water quality and health problems. As debris trapped in the substrate rots it produces ammonia and eventually nitrate. It may even serve as a breeding ground for parasites, harmful bacteria, and other harmful organisms. Lots of debris in the substrate can lead to infestations of Planaria worms, a type of flatworm. One of the biggest threats of pea-sized gravel is that once the goldfish are big enough, they can actually swallow the gravel. The gravel can get caught in the mouth, throat, or digestive system. There is little an aquarist can do in this situation unless the piece of gravel is in the mouth, but even then there is little that can be done and removing the piece of gravel can cause damage to the mouth and possibly lead to infection or permanent injury.
Another popular substrate is large gravel or pebbles. These do not pose the threat of ingestion that the smaller pea-sized gravel does. However, they can trap even more debris than smaller gravel, amplifying the effects of debris in the tank explained in the pea-sized gravel section.
Many people keep goldfish tanks bare bottom, without any substrate at all. This makes the tank very easy to clean. Many people find the look of a bare bottom tank too empty, too sterile, too much like a fish dealer’s tanks, or simply too boring. Although personal preference should not take precedence over the fishes’ health, the look of a tank can play a major part in how well the aquarist enjoys the tank, which is the whole reason they get it. In addition, since goldfish almost constantly sift through the substrate in search of food it is a very natural behavior for them and should not be denied from them.
When I see bare bottom goldfish tanks I just bad for the fish, it is boring and unnatural for them.
There is some debate as to whether or not sand can cause irritation to the gills. However, it does not seem like there is any actual fact to support this. So it does seem that this is simply a theoretical problem, not an actual threat. Sand is just as clean as bare bottom because it will not trap debris, instead it keeps all the debris on top. If there is enough flow in the tank then the debris will never even settle and simply keep moving in the water column until the filters do their job and trap it for the aquarist to remove.
Goldfish naturally sift through fine substrates in search of food. This is a very natural behavior for them that when offered the opportunity to take part in, almost all will. There are other concerns about risks with sand, things like toxic gasses building up, debris in the water damaging pumps, particulates in the water ruining clarity, etc. It seems that these problems have more to do with the type of sand used, not sand in general. Estes’ Marine Sand (also known as Ultra Reef and Stoney River) avoids these problems. So there is at least one great option for sand to use in a goldfish aquarium. Some people have had great success with commercial sands made for non-aquarium uses, like play sand or pool filter sand. It seems that these sands can vary in quality and characteristics, so not all are good choices. Many other people have had many of the problems mentioned when using these sands. For more information on using sand as a substrate please read the article Sand as a (Superior) Substrate.
My goldfish spend a substantial amount of time sifting through the sand. I think it is an important part of a tank that is setup to meet all their needs, physical and mental. I think anyone who keeps them on a bare bottom tank should watch them in a tank with sand and see how much they enjoy sifting through the sand.
In most cases live plants for the goldfish aquarium need to be carefully chosen and the options are limited to the species of plants that are too tough or taste too bad to be eaten by goldfish. Goldfish are herbivorous omnivores, so in general they are happy to eat live plants when they are offered. However there are many benefits of live plants in the aquarium. They keep water quality higher between normal water changes and provide oxygen while reducing ammonia, nitrite, nitrate, phosphate, and carbon dioxide. Live plants remove the nutrients that algaes live on, so they can help keep most algaes under control. They can also provide a food source for goldfish to graze on between feedings. As described in the diet section, when on a complete and balanced diet goldfish can be much less prone to destroying plants. So a proper diet to begin with can help keep the goldfish from eating live plants, which keeps them in the tank providing their benefits.
My goldfish were moved from a heavily planted 75 to an empty sand-bottomed 150. They explored at first but within a couple days tended to just hang out in the corner in a group. When I added live plants they immediately became more active. They were swimming through the plants and exploring again, the way they always did in the 75. I consider live plants to be a very important part of a goldfish setup, even if they just eat them.
For a complete guide to live plants, please read our Live Plants Article.
In general the decoration of a goldfish aquarium is completely up to the aquarist. It can range from a very accurate replication of their natural habitat, to something equivalent to an underwater amusement park. However, there are certain precautions that need to be made. In general, course or rough decorations, such as lava rock, should be avoided. These can cause cuts and abrasions that can lead to life threatening infections. Due to the shape and agility of goldfish, decorations that a goldfish may become stuck in should be avoided. For example, a ceramic bridge with an opening under the bridge may trap a goldfish that tries to swim under the bridge and through the ornament. Any decorations with sharp corners or rough areas should also be avoided. Even dense fake plants can become a trap for a fancy goldfish, so be aware.
Diet is a major issue with goldfish, and all fish. The most important aspect is providing a complete and balanced diet. There are a number of methods that people use in an attempt to achieve this.
Some people prefer a variety of natural foods such as live, frozen, dried, and fresh foods. Animal based foods like this can include things like brine shrimp, shrimp, earthworms, bloodworms, blackworms, daphnia, and even things like crickets, mealworms, and waxworms. Most foods found in the seafood section of the grocery store can be used as well. Many plant origin foods can be used. These can include skinned peas, bananas, blueberries, apples, blackberries, raspberries, melons, oranges, lemons, grapefruits, limes, strawberries, cucumber, zucchini, and carrots. Most of these need to be finely chopped, grated or sliced. Any foods offered should be bite size. Most of the foods’ skins are not digestible and should be removed before being offered to the goldfish. The limitation of this type of diet is that it is a lot of whole foods, but no trace elements and micronutrients that are provided in the various types of prepared foods. When compared to the ingredients lists of high quality prepared foods it can be shown that the foods that make up a ‘natural’ diet are equivalent to the main ingredients of the prepared food. What they do not cover are all the supplemental ingredients further down these lists that provide all the micronutrients and trace elements that are vital for long term health.
Flake foods are a very common type of fish food, but there are a number of problems with them. To begin with most are made for beginner aquarists and therefore are not of very high quality, but there are a number of high quality flakes. When foods are fed they immediately begin to lose water-soluble nutrients. Since all the food material in flakes is exposed to the water this can be a significant loss of nutrients. In addition, they fall apart quite easily and therefore make the water dirtier and when they are eaten a lot can fall apart in the fish’s mouth and end up coming out of the gills.
Pellets are a much better option for a prepared food than flakes. They do not fall apart the way flakes do. Pellets are in general of higher quality than flakes, but there are definitely a lot of low quality pellets available. There are floating and sinking pellets available. In round-bodied goldfish a diet of floating foods can contribute to buoyancy problems. This is because the extra air intake when feeding from the surface can cause air to be trapped in their digestive system. Fancy goldfish should be fed sinking pellets for this reason, which is described in more detail in the health issues section. The pellets should be small enough to be eaten whole. This prevents problems such as the pellets needing to soak for a while before the fish can eat them, at which point they are soft and can fall apart in the fish’s mouth and have small pieces come out their gills, contributing to problems described in the flakes section. Soaking foods removes water-soluble nutrients and therefore should always be avoided.
There is a reason why fish farms, aquariums, and scientists use pellets. They know the fish will get everything they need in every bite. They don’t waste time guessing ratios of generic frozen foods, making their unique foods, or anything else. They feed a complete and balanced diet in the form of a pellet (although obviously some are better than others).
Gel-based diets have proven to be a good option as well. Mazuri manufactures a gelatin diet that is available online and in some stores, frequently by special-order only. NLS now also has a gelatin food, Nutri/Gel. In addition to this there are homemade gel-based diet recipes available online. The benefit of this option is that it can be customized as the aquarist desires. This allows the aquarist to include whatever foods items are on hand and in season, as well as add vital supplements. However, this also forces the aquarist to be an amateur nutritionist. Since nutrition is arguably the most important factor for long term health leaving it to an amateur guessing and hoping is not in the best interest of the fish.
Another very popular method is the mixed diet. This diet uses one or more high quality pellet foods as the staple of the diet. In addition to this, certain frozen foods and even fresh produce can be used. In essence this diet combines prepared and ‘natural’ diets to create what many feel is the best diet. This diet provides variety, but this is not necessarily complete and balanced nutrition.
The Variety Myth:
Classically one of the most common guides for fish diet is ‘variety, variety, variety’. This has been a good guide for a long time. A complete and balanced diet includes all the macronutrients as well as the needed micronutrients and trace elements vital for long term health and proper physiological function. In the past variety was needed to provide all of these nutrients because there was not a food that when fed exclusively could provide a complete and balanced diet. This is no longer the case. There is now at least one brand of fish food that even when fed exclusively provides a complete and balanced diet without the need for any supplementation or variety, this brand is New Life Spectrum. As described in the ‘natural’ foods section variety is usually a bunch of main ingredients, not the needed supplemental ingredients.
Some people prefer to presoak foods to ensure they sink and that all air is removed from the food. The problem with this is similar to what was explained in the flakes section, many water-soluble nutrients are removed. If the food is naturally floating it can take a long time to be soaked enough to sink, which will remove a lot of nutrients. If the food naturally sinks then there is no need to soak to remove air. If the food contains too much air and is causing buoyancy problems then finding a new food is the solution, not soaking the current food.
I have kept my goldfish on New Life Spectrum exclusively. I believe this has played a major part in why they do not destroy the plants. When an animal is getting a complete and balanced diet they will not have the same drive or craving for certain foods the way they would if they were not getting everything they need. I believe this is exactly why my goldfish do not eat plants that almost every other goldfish is more than happy to eat. I think this is more than just a coincidence that all of the goldfish I have ever fed New Life Spectrum to have not shown the usual preference for plants. There are other diets that have achieved the same results in this aspect. The one that seems best is a homemade gel-based diet that includes the proper variety of ingredients needed to fulfill all dietary needs.
For articles on New Life Spectrum, Fish Nutrition, Myths About fish Nutrition, and more please see the Feeding and Nutrition page.
There are a number of health issues that goldfish are more prone to than other types of fish. It is important to understand these issues completely so that you can have the best chances of completely avoiding them.
Buoyancy problems can have a number of causes. The round body of fancy goldfish compresses their internal organs into an unnatural arrangement. This can prohibit them from functioning properly. This is what makes them more susceptible to these types of problems. The two main things that can cause buoyancy problems are problems with the swim bladder and problems with the digestive system.
Swim Bladder Problems:
The swim bladder is an air filled organ that in many fish, including goldfish, has a duct that connects it to the mouth so the fish can quickly exchange air with the atmosphere at the surface of the water. In round-bodied goldfish this duct can be compressed in a way that prevents the proper exchange of air with the swim bladder. The main function of the swim bladder in fish is buoyancy and orientation of the body. Water quality problems make the swim bladder and duct more susceptible to infection. In cases when infection occurs the fish will usually have visible problems maintaining neutral buoyancy and a normal orientation in the water column. The fish may float or sink and/or have problems remaining upright. For example, the fish may always face head down, or be completely upside-down.
When there are swim bladder problems, water quality should be immediately improved and maintained that way permanently. This simply means more and/or larger water changes. In many cases this is the only change that needs to occur to cure the problem. In harder to treat cases certain medications may be needed, but will vary depending on exactly what is causing the infection to begin with (bacteria, fungus, etc.).
Some cases cause too much permanent damage to the swim bladder and the fish will never be able to maintain proper buoyancy and/or orientation. These may be considered ‘special needs’ fish. Some people have even gone as far as to make harnesses using thin cloth strips, fishing weights, and fishing bobbers to provide the neutral buoyancy and proper orientation the fish needs.
Digestive System Problems:
The compressed condition of round-bodied goldfish can also cause problems with the digestive system. When they are fed floating foods this can cause them to take in too much air, which can be trapped in the digestive system. This can cause buoyancy problems that may look identical to swim bladder problems. Some believe that the floating foods can cause problems in the swim bladder, but it seems more likely that in these cases the problem is actually in the digestive system but the symptoms match that of swim bladder problems.
In most cases a change in diet to strictly sinking, high quality foods is all that is needed to cure the problem and prevent it in general. However, in some cases infections develop that can kill the fish within a couple days. In these cases there is little that can be done because the condition has become so severe. Medicating the fish may help with the infection. Certain foods like skinned peas can be used to flush out the digestive system of any air.
Many people have the idea that soaking foods so that they will sink is a good option, however this removes many water-soluble nutrients that are vital for the fish. Soaking the food may prevent the immediate problem of air in the digestive system, but it can cause dietary deficiencies long term.
Dropsy and Pop-eye:
Dropsy is not a disease, but simply a symptom of many types of problems. Dropsy is the swelling of the internal body cavity where the organs of the gut are located and/or the tissue of the skin where the scales are rooted. If it is only in the gut the fish will simply be swollen in the main trunk of the body. If it is in the skin, the swollen tissues can cause the scales to project out, causing what is called ‘pine coning’. Either condition is more obvious when the fish is observed from above. This can be a symptom of anything that inhibits the ability of the kidneys to properly remove water from the body. In freshwater, water is constantly trying to enter the body due to osmotic pressure. With more solutes in the body (ions, salts, etc.) than in the water, the water is constantly going into the body in an attempt to equalize the concentration of solutes inside and outside of the body. When the balance of water going in and water being taken out by the kidneys is thrown off, the body swells. Water usually gets in through the gills, but can also get in through injuries or infections that open tissue to the water. Infections of the kidneys can prevent them from removing excess water from the body. This can be caused by a virus, bacteria, fungus, or even parasites.
In most cases where dropsy is noticeable, it is too late and it is rare that the fish lives. When they do, aggressive treatments are usually needed. Lots of salt in the water (more than one tablespoon per five gallons) helps increase the amount of solutes in the water, reducing how quickly water enters the body. In addition an aggressive use of strong medications can help save the fish.
Pop-eye is basically the same as dropsy, except that it occurs in the area behind the eye and not the main cavity of the body. This causes the eyes to project out from the head. Medications seem to be more effective with pop-eye than with dropsy, but there is still no guarantee and it is a difficult problem to deal with. In many cases of saving the fish’s life, too much damage has been done and the fish loses its sight, the eye being visibly collapsed.
PERMANENT/GENETIC BUOYANCY PROBLEMS:
Because of the way goldfish have been bred (very aggressively and selectively for certain traits while overlooking hardiness and health) many have chronic or genetic problems with buoyancy. The super-compressed nature of some breeds increases this problem. The fish may be otherwise healthy, but spend any time not actively swimming just sitting on the bottom in the corner, or floating upside-down on the surface. Others are super sensitive to things like water quality and diet. There are even some that cannot tolerate any air in their food, even the little amount found in sinking pellet foods. These fish may need a gelatin based diet or other special homemade foods that prevent any air from entering the digestive system, but these cases are very rare.
Goldfish are native to waters that range in pH from 6.0 to 8.0 and hardness from soft to moderately hard. This puts them at ideally around a pH of 7.5 or so. However, they are also very hardy and very adaptable. This means that in most cases there really does not need to be any steps taken to alter the pH or hardness for goldfish. Outside of extremes in chemistry, it is best not to try and correct the parameters and rather to focus on water quality. Stability in parameters and high water quality will achieve better results and are much better for the fish. There are many chemicals involved with water quality, but the most important one to monitor is nitrate concentration. Ideally the nitrate concentration will be maintained at no more than 20ppm. The other chemicals involved in water quality generally correlate with nitrate concentration. So as long as the nitrate concentration is maintained with a moderate to aggressive water change schedule, the water quality should be very good.
For more information about freshwater chemistry please read the Chemistry in the Freshwater Aquarium article.
Salt use with goldfish is also a highly debated topic. Many use it all the time, citing the believed benefit of preventing diseases. This is actually a bad thing because even if it prevents diseases, which is not well supported, when diseases do come up salt is ineffective against the pathogen because it has been made resistant to salt due to its constant use in the aquarium. Others claim that salt helps ease osmotic pressure and reduce stress. These are freshwater fish and therefore have excretory systems capable of dealing with the osmotic pressure of freshwater. In some cases salt use may be necessary. When treating certain diseases it can be an effective tool. In cases where fish are already stressed, such as in most local fish shops, salt can help reduce stress and prevent disease, but this should only be used short term. In the aquarist’s home salt could be used in a quarantine tank to ease the stress of a new environment on the new fish.
The recommended dosing, usually one tablespoon per five gallons, is also very high. Lake Malawi is one of the saltiest bodies of freshwater in the world and the dosing of special salts for fish from this lake is only one tablespoon per forty gallons. That means the most common recommended dose is eight times the concentration of one of the saltiest bodies of freshwater in the world.
I have tried both methods and see no difference or advantage in using salt. As stated it may help if the fish is stressed, but in general it is not needed. The constant use of salt can even cause problems long term.
There is some debate about certain possible tankmates to go with goldfish. There are a number of concerns that dictate what can be a tankmate with goldfish.
One major issue is the temperature. Goldfish are usually kept in room temperature water. This is much cooler than most other fish will do well in since most are native to more tropical regions and prefer something in the upper 70s to lower 80s. Some species’ temperature ranges may barely overlap with goldfish. This usually means that the cooler temperature is barely in their range and long-term exposure to this can cause stress on the fish and cause them to fail to thrive or even cause major health problems long term. So species that are also native to sub-tropical regions need to be chosen.
Many people cite water parameters as a limiting factor for goldfish compatibility. This is true to a point, but not as big of a concern as many make it out to be. There are water parameters like pH, GH, KH, etc. that should be in specific ideal ranges. However it is not essential that they are in specific ranges. For example, a certain species of fish may have a listed pH range of 7.6-8.0 with an ideal GH (measured in degrees hardness, dH) of 15. But even if the tap and tank waters’ pH are 7.0 and the dH is 8, this does not mean that anything needs to be done to alter the parameters. It has been shown repeatedly how high water quality and stability of parameters, not specific parameters, is better than trying to ‘fix’ what isn’t even broken by using chemicals to adjust the pH or hardness. Trying to alter pH and other parameters can cause an up and down or roller coaster effect that needs constant adjusting and is more stressful to the fish than having a stable yet ‘wrong’ parameter. There are few cases where the parameters really should be altered and this is usually only in situations where fish from one extreme are attempting to be kept in water of the other extreme, like discus being kept where the tap is 8.2. In general pH, GH, and KH all correlate. There are some cases where GH may not, but since KH controls pH they will always strongly correlate. A high KH keeps the pH high. When it comes to compatibility of tankmates, the ideal readings do not need to match perfectly as long as extremes are avoided. Since goldfish prefer harder, more alkaline water tankmates should not be native to soft, acidic waters.
There are certain behavioral aspects that need to be considered for tankmates for goldfish. Goldfish tankmates cannot be too aggressive or nippy. Fancy goldfish are relatively slow, some breeds significantly so, and are easy targets for aggressive and nippy fish. Tankmates should also not be too small or slow otherwise they may become food for the goldfish. In my experience mollies and swordtails tend to be too nippy. The biggest and healthiest platies I have ever seen are in with my goldfish, and it is rare that one ends up being too nippy for the goldfish. So although livebearers love the hard water, they are not usually a good tankmate (except for platies). Small fish like danios and White Cloud Mountain minnows are very good overall, but every once in a while a goldfish will figure out how to catch them. My school of zebra danios quickly went from over 30 to about five when at least one of my goldfish figured out how to catch them.
Misconceptions and Myths:
There are a number of common, yet false, myths and misconceptions about goldfish when it comes to compatibility with other fish. Other fish being tropical does not inherently make them incompatible. Goldfish can thrive in a wide range of temperatures, and not all fish thought of as tropical actually are. Some people claim that goldfish are too messy, dirty, or produce more waste than other fish. Goldfish produce no more waste per weight than other fish. Goldfish just happen to be larger and bulkier, so it would take many more neon tetras, for example, to equal the weight of a goldfish. This larger school of neon tetras produces about as much waste as the single goldfish of equal weight.
Some people believe that the goldfish has a less efficient or more wasteful digestive system. This is not true either. This seems to have more to do with the quality of the food to begin with rather than their digestive system. Many foods have a lot of fillers and other ingredients that are not easily digestible. These produce more waste in any fish they are fed to, not just in goldfish. There are some fish that are a little dirtier than others simply because they are larger or because of feeding habits. Cichlids and goldfish are two types of fish that seem to be slightly messier than some others. In addition, goldfish kept in tropical temperatures will have a faster metabolism and produce more waste than they would if kept in a cooler temperature. This may also contribute to the belief that they are dirtier than other types of fish when people have not been keeping them in the proper conditions. There is even the belief that the slime coat of goldfish is toxic to other fish. This has no factual support at all and considering how many people have kept them together without the other fish dying or showing any ill effects shows that this is not true.
Good options for tankmates:
There are some species that have proven to be great tankmates for goldfish. These fish have compatible water parameter and temperature needs. They also have behaviors that make them very compatible with goldfish. These fish include, but are not limited to: White Cloud Mountain minnows, dojo/weather loaches, platies, all varieties of zebra danios (wild/zebra, gold, blue, leopard, longfin, and Glofish™), pearl danios, scissortail rasboras, hillstream loaches, some barbs, and many more. These are the best options, but are not guaranteed to be successful tankmates in every situation. In order to determine if a fish will be fine at the temperature of your goldfish tank, check it on Fishbase.org. It is used and maintained by scientists so it is more accurate than hobbyist sites and even most books.
There are many breeds of fancy goldfish. There are some very important differences between them that must be taken into consideration before they are chosen. In general certain breeds are hardier or more sensitive than other breeds. In general the order from most hardy to least hardy is roughly: fantail, telescope eye (including black moors), ryukin, oranda, lionhead, ranchu, celestial eye, bubble-eye, and pearlscale. There are many factors that can affect this, as well as other breeds and breed characteristics. Breeders can increase or decrease hardiness and adaptability. The water parameters the fish were born and kept in previously will affect their range of tolerance. This is a rough guide to be used as a general idea of the order in which a goldfish keeper should progress. It is not recommended to simply jump into the more sensitive breeds without any experience with the hardier and more forgiving breeds.
Be aware that goldfish can lose or change their coloration over time. They are white, under orange, under black. The black is very easily lost. It usually only holds well in outdoor ponds under sunlight. Indoors it usually fades away completely. Black moors may hold it, but may also drop some of it and produce a cool blotchy orange and black. Calicoes tend to hold it well too. It may still change, but they do tend to hold it. Orange can reduce over time as well, but not usually nearly as significantly as black. If they are injured (such as a torn fin) new tissue may come in black. This is normal. It will probably be lost like usual. Remember, the fish in the stores were probably in outdoor ponds just a couple weeks ago, so they may have the best coloration they will ever have. Goldfish are probably the fish most impacted by lighting.
I have now accidentally bred both pond and fancy goldfish in an aquarium. Both times the fish were being fed New Life Spectrum exclusively. In both cases the temp was above room temp. In the case of the pond goldfish it was about 82-84F and triggered by a cooler water change. In the case of the fancy goldfish it was about 78F and again triggered (if I remember correctly) by a cooler water change. I was able to collect some of the eggs from the fancies and put them in a separate ten gallon with a sponge filter and heater where I raised them on Golden Pearls and then New Life Spectrum.