Myths About Goldfish and Their Care

There are many myths about goldfish and their care. Most seem to be endlessly repeated without thought or investigation. With very little, simple research most should at least be seen as not as cut and dry as many books and people may lead you to believe. Goldfish are one of the most common pet fish in the world and are prized for anything from a backyard pond or a child’s first pet to show quality goldfish worth hundreds and hundreds of dollars. This popularity makes it even more important to make sure that the information that is out there is accurate.

Goldfish are Coldwater

This isn’t completely a myth, goldfish are definitely well adapted to temperate climates, commonly called cold water. They can live in water down to the freezing point, as long as it stays liquid. The myth part of this is that they can only be in cold water. This is not true. They are actually eurythermal, which means they can thrive in a wide range of temperatures. has their temperature range listed as 0-41C (32-105.8F). (1)

It is important to distinguish between truly coldwater fish and those who can thrive in warmer temperatures as well. Truly coldwater fish are species that cannot thrive in warmer temperatures. The increased temperature causes stress, failure to thrive, and can even kill them. This would include species like trout and white cloud mountain minnows. These are species that when in higher temperatures show signs of stress such as reduced coloration, overall failure to thrive, and more extremes such as severe stress, illness, and death. This is not the case with goldfish. This is not a case of goldfish simply tolerating higher temperatures than they are given credit for, they thrive in these temperatures just as well as they do in lower temperatures. In higher temperatures they do not show these signs of stress and failure to thrive. They can thrive in higher temperatures.

The countries that goldfish are native to are: China, Hong Kong, Japan, Laos, Macau, and Myanmar. The countries that goldfish have been introduced to and is now naturalized in (maintaining populations) are very numerous and include: Afghanistan, Argentina, Australia, Bolivia, Brazil, Chile, Colombia, Costa Rica, Ethiopia, India, Indonesia, Iran, Iraq, Korea, Madagascar, Malaysia, Mexico, Peru, Philippines, Puerto Rico, Samoa, Saudi Arabia, Singapore, South Africa, Thailand, Uruguay, U.S. Virgin Islands, U.S.A. (including Florida and Hawaii), Vietnam, and Zimbabwe. (2)

The map below shows the natural and introduced distribution of goldfish in the wild. The blue countries are where they are found naturally. The red countries are the ones they have been introduced to and have established populations.

goldfish with tropical fish

The CIA World Factbook lists China as varying from sub-arctic in the North, to Tropical in the South.(4) Their natural range includes the Tropical Southern region as well as Hong Kong and Macau. The same source lists Laos as having a Tropical climate.(4) You can see how tropical of a climate their natural range reaches, less than 15 degrees from the equator in Myanmar, Thailand, and Laos.

There are some significant implications in the range included in the introduced range of the goldfish. Throughout Indonesia and Malaysia it sits right on the equator. It also stretches continuously from India through the Middle East and into Saudi Arabia. Africa also shows their distribution within 30 degrees latitude. One of the most significant introduced areas is South America. Almost all of South America is now the introduced home of this supposedly coldwater species. This is not only significant in terms of latitude and temperature, but these are the exact same waters so many of the hobby’s tropical fish come from. Goldfish in the wild are literally swimming alongside the exact species kept in tropical tanks, and yet it is claimed to be too hot for them. In addition to all of this is their distribution in Mexico and into the United States. In the U.S. they are not limited to temperate Northern regions, but are present in tropical areas such as Hawaii and Florida. They are not just in large bodies of water that will have a relatively stable temperature throughout the year and day, but they thrive in small ponds that will quickly rise in temperature under the hot summer sun in Florida.

These are not the coldwater fish we have thought of them as. Beginner books list them as coldwater and frequently give temperature ranges to match this. So it is no wonder people think of them as coldwater fish, we are told they are from the very beginning. But the evidence simply doesn’t support this.

There is even some indication of higher temperature being beneficial in multiple ways. It has been shown that pigmentation is increased with higher temperature. One study found that the ideal temperature for pigmentation was 26-30C (78.9-86F). (5) Pigmentation (coloration) is generally considered one indication of overall health and ability to thrive. In a different experiment it was found that the highest growth rate was at 28C (82.4F). (6) In both of these studies the ideal temperatures were found to be the highest temperatures used in the experiment. Since they did not use even higher temperatures it is unknown based on these experiments if even higher temperatures would have produced even better results.

Numerous goldfish keepers have found that higher temperatures do not have negative effects on goldfish, some even cite their goldfish thriving in temperatures as high as 90F. Many of the fancy varieties are bred in tropical climates such as Hong Kong and Thailand. Some keepers have even noted significant stress in these fish when moved immediately into cooler temperatures upon being imported. It is recommended that fish bred in warmer regions are kept in higher temperatures, at least a few weeks, when they are first imported to reduce the stress of moving from a warmer to cooler climate (air conditioned American homes).

Author’s experience and conclusion:

I have had goldfish in the same tank as discus and other tropical fish. I used them to help control the duckweed and crystalwort that I now regret introducing to one of my tanks. These are the same goldfish I usually keep in the room temperature goldfish tank. I see no signs of any issues while they are in the tropical tank. They are just as active, if not a little more active. They are just as colorful. They eat just as well. I have seen no signs of any problems with keeping them in this temperature. I have talked with many other goldfish keepers who have or have had goldfish in tropical temperatures. From ponds in tropical areas like Florida and Thailand, to having them in tropical tanks in general.

But what does this mean for the hobby of keeping goldfish? We do not need to go buy heaters for our goldfish tanks. They thrive in room as well as tropical temperatures. What this means is that when people ask what they can keep with goldfish, we need to focus on the real problems with goldfish in the same tank as other fish and not just regurgitating the myth of temperature problems. This does not mean we can just start putting goldfish in tropical tanks. Goldfish are not compatible with most tropical fish. Almost all are too aggressive, too nippy, or too small to be in with goldfish.

1 Carassius auratus

2 Countries of Carassius auratus

3 World Map in black and white (color added by author)

4 CIA The World Factbook

5 Aquaculture Nutrition, 2005, 11 p19-23, “Effect of microalgal biomass concentration and temperature on ornamental goldfish (Carassius auratus) skin pigmentation”. L. Gouveia, and P. Rema.

6 Aquaculture, 1995, 136 p341-349, “Influence on feed supply, temperature and body size on the growth of goldfish Carassius auratus larvae. P. Kestemont.

Goldfish Need Salt

This is a pretty common myth about goldfish. Salt use in all freshwater aquariums is debated, but it has been shown to not be beneficial. Long term use can actually cause problems. But its use with goldfish seems to be even harder to get accurate information on. Many use it, sometimes quite heavily, all the time. The general ideas are that it reduces stress by reducing osmotic pressure, helps prevent infections, provides minerals, protects from nitrite poisoning, and to stimulate slime coat production.

Minerals are provided by tap water in most cases. In addition to this, a truly complete and balanced diet will provide all needed micronutrients and trace elements.

Using salt to prevent infections just means it is useless as a treatment for any disease that develops during constant salt use since the pathogen has acclimated to the presence of salt. Many pathogens only need salt treatment to be cured, and constant salt use will make these treatments useless and ineffective, requiring the use of riskier, more toxic medications.

Nitrite should never be a problem. The aquarium should be cycled before fish are put in. Once cycled the tank should never go through any mini-cycles unless the keeper either cleans the filters too well (killing the nitrifying bacteria) or adds too many fish at one time. If problems do arise the best solution is water changes to get rid of the nitrite.

Fish have no problem producing adequate slime coat. This is a very natural process for them. It is thought that if salt does indeed increase slime coat production it is because the salt is actually irritating the goldfish and the physiological response is to protect itself with more slime coat.

These are freshwater fish. They have evolved to live and thrive in freshwater conditions. This means their kidneys have evolved to function in freshwater. Adding salt will not help this.

In addition to the reasons for salt use not being true, the dosing is even worse. Special salts are available for cichlids of Lake Malawi, one of the saltiest bodies of freshwater in the world. The dose for this salt is one tablespoon per 40 gallons. The usual recommendation of salt for goldfish is one tablespoon per five gallons. That is eight times the concentration of one of the saltiest bodies of freshwater in the world.

The natural water conditions of goldfish do not include brackish water at all, they are only listed as freshwater fish. (1) In addition, goldfish are considered stenohaline (narrow range of salt tolerance) as opposed to euryhaline (tolerant of a wide range of salinities). In an experiment it was found that with goldfish, the higher the salinity was the higher the production of ammonia and urea. (2) They literally produced more waste with more ammonia because of the higher salinity.

A different study found similar results. This study found that lower salinities (6ppt and under) were not significantly stressful, but higher salinities produced significant muscle dehydration, adverse affects on growth, reduced food intake, and reduced food conversion rate. In addition to this, the study found that diurnal activity was significantly lower in all groups treated with any level of salt as opposed to the group maintained with no salt. (3) However, this study only lasted three weeks. This means that long term even the lowest salinities could possibly produce significant problems that were not produced in this very short trial period. The fact that the other indicators did become apparent in only three weeks shows how damaging constant salt use can be.

These are freshwater fish, plain and simple. They come from freshwater, just like all other freshwater fish. These are not brackish fish that can handle freshwater, these are true freshwater fish. Constant salt use has significant negative effects on this species. Its use should be limited to directly treating illnesses and ailments.

Author’s experience:

When I first got back in to goldfish after not having them for too many years, I was told how great and important salt was. So I followed the instructions by the employee at the pet shop that is the best in the area for koi and fancy goldfish. I salted the tank, quite heavily, and maintained the salt level the entire time I had that tank setup. One fish grew very well, but in general they did not do as well as the ones I now have. I have made even more improvements with the ones I have now, so it is not due just to the salt, but I definitely see no need for it at all. I have talked to many goldfish keepers who have found the same results, that the idea of salt use with goldfish is really just a myth. For treating illnesses it is a great option, but not good for constant use.

1- Carassius auratus

2-Aquaculture, 2004, 238 p.499-507, “Excretion of ammonia and urea by phylogenetically diverse fish species in low salinities.” I. Altinok, and J. Grizzle.

3-aquaculture, 2008, 276 p.171-178, “Growth, food intake regulation and metabolic adaptations in goldfish (Carassius auratus) exposed to different salinities.” R. Luz, R. Martinez-Alvarez, N. De Pedro, M. Delgado.

Goldfish are Dirty and Produce More Waste than Other Fish

This is also not true. Goldfish do not produce any more waste per weight than any other fish. There are two things that generally determine how much waste a fish produces, the quality of the food and the normal respiration based on metabolism.

The quality of the food being fed can affect waste production. If a food is of lower quality then there are more fillers. These are cheap ingredients to add bulk to the food. These are not digestible and simply pass through the goldfish. Many of these contain protein that is not bioavailable to the fish but is used to boost the protein value of the food overall in a lab. Any protein that passes through the goldfish and is not digested will be broken down by bacteria in the water into ammonia and, in the end, nitrate. This will decrease water quality. Sometimes the physical nature of the food affects how clean it is. Some fall apart very easily and therefore when eaten produce ‘dust’ that will simply feed the aforementioned bacteria and, again, in the end increase nitrate.

The other aspect is metabolism. Cells produce waste. The more cells there are in the fish the more waste that will be produced. This is just a factor of mass. This waste is excreted as ammonia (by the gills and kidneys) and as urea (by the kidneys).  Per weight goldfish do not produce more waste than any other type of fish. A higher temperature will increase the rate of metabolism and therefore the amount of waste produced, but this is also true of every other fish.

Goldfish Do Not Have a Stomach

This is also not true. They do have a stomach. The reason many say it is not there is because it is relatively small. There are a number of reasons for this. These are based on the main functions of the stomach: hold recently ingested food, provide physical breakdown of this food, and begin digestion with acid and enzymes.

Goldfish are herbivorous omnivores, this means that they are omnivores that tend to feed more on plant than animal materials. Their stomach has evolved to match this. Naturally they will take in small amounts of food at a time, but very frequently throughout the day. They constantly feed on small amounts of food at a time. Because of this they do not need a stomach capable of handling very large volumes of food at any given time. The opposite of this would a large piscivore, for example the jaguar cichlid. They eat large volumes of food at a time but do it occasionally. This means their stomach needs to be able to handle this large intake of food, so their stomach will be larger.

The second function of the stomach is to physically break down the recently ingested food. However, in goldfish this function has already taken place by the time the food reaches the stomach. The pharyngeal teeth are plate-like teeth in the back of the throat of the goldfish. These provide the physical breakdown of food before it gets to the stomach. So this function of the stomach is already fulfilled.

The last function is to begin digestion with acid and enzymes. This does not require a large stomach. The stomach acid and enzymes can do their jobs on the small volumes of food present in the stomach within the small volume of the stomach. The fact that so little food is present in the stomach at any given time aids in this.

So the stomach can do what is required of it in its current reduced form in the goldfish.

Related to this myth are myths about goldfish not being able to process protein, that they cannot process food in general very efficiently, or that they cannot absorb certain nutrients because of their reduced stomach.

The stomach does not absorb nutrients, this process occurs in the intestines. They can process protein very well, otherwise they would have a very slow growth rate, yet they are very fast growers when cared for properly.