Fish Nutrition

Diet is the most important aspect of animal care. In aquariums it is matched only by water quality. The diet fed to a fish will dictate its health and long term ability to thrive. The fish’s body can only work with what it is given. If given a poor diet the fish is very prone to illness, more sensitive to any stressor, and much more likely to develop physiological problems over time. In addition, low quality foods contribute to lower water quality since there are more fillers (more food has to go in for the fish to get the same amount of nutrition, and more comes out the other end).

Water quality is as important as diet. Poor quality in one of these aspects of care can undo the highest quality in the other. These are the two main things that can be controlled in an aquarium. Water quality is pretty straight forward and easy to control. Diet is more complicated. The three main types of nutrients are macronutrients (protein, fat, etc.), micronutrients (vitamins and minerals), and trace elements (molybdenum, zinc, etc.).

When determining the diet of a fish the digestive system of the fish needs to be taken into account. Fish have evolved to a certain diet over thousands of years (generally much longer). There are many variations that can occur to a digestive system based on the diet. In general more carnivorous fish have shorter, simpler digestive systems. Fish that are more herbivorous have longer and more complicated digestive systems.

The more specialized the digestive system is the more limited the fish is in what it can properly digest. This can be misleading to many aquarists. This does mean there are more things that the fish cannot eat, but it does not mean that what the fish eats naturally is the only thing it can eat.


Variety has been considered one of the most important aspects of a good diet. The idea is that anything lacking in one part of the diet will be made up for in another. This has proved to be a good method for a long time, but it is not necessarily a vital part of a diet.

Variety is not a nutrient. It is not a fat, protein, or carbohydrate. It is not a Vitamin or mineral. It is not a trace element. Variety is not needed for any physiological function. Variety was one good method of helping to ensure a good overall diet. But the real goal of any diet is truly complete and balanced nutrition. This does not require variety. It requires all the major and minor nutrients, as well as all needed trace elements. In addition, they must be in the proper proportions and ratios. More and more animal keepers are moving away from ‘natural’ diets and relying on prepared foods to meet the nutritional requirements of animals. This is partially due to the fact that prepared foods are generally increasing in quality overall. Aquariums are using gel foods. Zoos are using pelleted diets. Veterinary manuals recommend the use of pellets, nuggets, etc. to ensure an animal receives every nutrient it needs and cannot pick through an assortment for its favorites. Bird keepers are switching to pelleted foods and leaving seed mixes behind. Pelleted foods ensure that every bit of nutrition in the food gets into the animal in every bite. But these prepared foods do have a wide variety of ingredients, so it can easily be argued that the variety has shifted from being provided by us, to being provided by the manufacturer of the diet. But physical variety, as in what the fish actually receives, is not vital.

Mixed diets like seed mixes for birds allow the animal to pick through and choose the items it likes the most. The problem with this is that there are certain nutrients animals like more than others. In the same way that humans desire foods rich in fat and sugar like ice cream and fried anything, animals have a natural drive for certain nutrients. In the case of birds and seed mixes, birds are attracted to fatty seeds. When given the option they will feed on these. In the wild this is a very good instinct. Foods like these can be rare, possibly only available a few weeks out of the year. Because of this it is important for the animal to eat as much as possible when the food is available. The same goes for humans. This instinct that drives us to fill up on fried food followed by ice cream helped us get through the leaner times our ancestors faced. The problem is that those are no longer the conditions we live in. Those nutrients are no longer rare, but the drive for them is still strong.

These same concepts apply to fish. Certain foods are rarely available. For example, in the Amazon River system when the rainy season begins, many terrestrial and even arboreal species (especially invertebrates) are washed into the water. This provides a lot of nutrition suddenly available to the fish. These animals can be much higher in nutrients like protein and fat. This sudden increase in nutrient availability is one trigger (sometimes the most important) in beginning the breeding season. In many cases simulating this in the aquarium is all that is needed to trigger breeding. It is during this time of rare availability that fish fill up on these types of food, getting nutrition not available any other time of the year. This natural high drive for such foods can lead to poor overall diet when those foods are available all the time. If offered all of the time (not necessarily every feeding, but throughout the year, not just to trigger breeding) the fish can fill up on these foods and become obese. This unbalanced and incomplete diet can cause problems long term. There is also evidence that a constant supply of food with an overall fat level higher than 5% can actually cause fat deposits to develop throughout the body. This can cause health problems long term. This could be a major cause of ‘mystery’ deaths, ‘old age’ deaths, etc. Since necropsies are effectively never done on aquarium fish it is impossible at this time to know how bad problems with diet truly are.


A diet is nothing more than its ingredients. Without high quality ingredients there is no high quality diet. The ingredients list is an important thing to check when deciding what kind of food to feed. However, many things about it can be misleading. For example, meals used as main ingredients can vary greatly in quality. These meals (such as ‘fish meal’ and ‘krill meal’) can be made out of the entire animal or only by what is left as scraps after the animal is processed for human consumption. Whole animal meals are very healthy for fish and provide a lot of nutrition. Scrap meals are very lacking in proper nutrients and should be avoided. In most cases it is not listed on the packaging whether or not the whole or only parts of the animal are used. To find this out contacting the manufacturer via email or phone may be required.

Be careful of buzzwords. Many companies are very aware of the types of ingredients the average aquarist thinks are good or important. Because of this those types of ingredients can come up in ingredients lists, sometimes very far down in the list. This means there is very little to effectively none of this ingredient in the food, but it can still be marketed as containing that ingredient in big bold letters on the packaging. In addition to this, the company may simply use parts of that ingredient and not the whole thing. This again is low in quality but still allows them to market their food as containing it.

Another important consideration with diet is the use of chemical preservatives. By law, meals containing fat brought into the U.S. must contain ethoxyquin. Some companies have it listed in their ingredients because it is in at least one of their meals, other companies add it to the food as a whole. In addition to this some companies add other preservatives to the food. Keeping these chemicals to a minimum can help maintain long term health.

The nutritional values listed on the package can be very misleading. One of the most common problems is the protein level. In many foods this can be very high. However, a lot of that protein may be in ingredients that are not easily digested by the fish, some are not digestible at all. This can lower the bioavailable protein. The protein level is measured in a lab which can measure all the protein in the food and not just what is actually available to the fish. So even though the protein in the food may be 45%, the protein actually available to the fish may only be 20% or less. In addition to this problem, all of that other protein that is not available to the fish simply passes through the digestive system. This can cause multiple problems. In the fish this can cause some digestive problems (like Malawi bloat in mbuna cichlids). Once it is passed through the fish and discharged into the water column it can be broken down into ammonia and eventually nitrate, lowering water quality. So high quality food is not just good for the fish, but also for the health of the whole tank.

Another very important consideration is the use of artificial colors in foods. It is thought that these can contribute to increased coloration in the fish, but this type of unnatural color enhancement should be avoided. The long term effects of these chemical colors is unknown, so it is not necessarily safe in general. Companies know what unsuspecting consumers want. It is expected that ‘spirulina enhanced’ and ‘veggie’ varieties will be green. Because of this many companies dye these foods green, even though they are not green naturally, at least not nearly as green as they are after being dyed. Carnivorous foods are generally expected to be some shade of red. So again, dyes are used to get the food to match the consumer’s expectations. This is evidenced most in the fact that these types of foods may not have very different ingredients (such as spirulina being far down in the ingredients list and the rest of the ingredients are identical or almost identical to the rest of the company’s formulas but the food is still very green). Comparing the different formulas and seeing very different colors but little variation in the formula itself shows that most likely the variation in color is almost completely reliant on the dyes, not the ingredients. Another reason some companies dye their food is to hide any variation in color caused by different batches of ingredients. The color of a food can change based on the color of its main ingredients. This can be caused by the time of year (for example, krill are colored more intensely at certain times of the year) or by the particular batch (krill feeding on one food may be colored differently than a batch that had been feeding on another food). This type of variation can seem like low quality to the consumer who doesn’t know any better. To avoid this doubt the company simply dyes the food (frequently to match the colors on the packaging). If a company is not confident enough in its food to let you see it before it has been dyed, don’t assume the nutrition was that good to begin with.

How Much and How Often?

Frequency of feeding and amount of food fed per feeding is also an important issue. To determine how much and how often to feed a few things need to be considered. Most importantly is the species in question. Many fish would feed almost continuously throughout the day. Most community fish and most herbivorous fish would naturally eat throughout the day, either almost continuously or very small meals very frequently. To simulate this multiple smaller feedings throughout the day are better than one large meal. Large predatory fish generally eat large meals very infrequently. For example, an oscar or jaguar cichlid may only eat once or twice a week, but they will eat a lot at each of those meals. For these fish it is best to feed large feedings once or twice a week. The best time for this is before a water change. This way any extra food is easily cleaned up, and since a fish’s metabolism is increased slightly while digesting such a meal they will also produce more waste during this time. So feeding these fish a day or so before a water change is ideal. Age can also affect feeding frequency and amount. Fish that are not full grown eat more and eat more often. So although an adult Oscar may only need to eat once a week, juvenile Oscars need to be fed many times throughout the day. At a young age they would naturally eat the same way community fish will, many small meals throughout the day, constantly foraging for food.

The other important consideration is the stocking of the tank. Tanks that are heavily stocked or have many different types of fish usually have very aggressive feeders, and some more skittish feeders. In these situations it is usually better to feed more food per feeding. This can help ensure that the more skittish feeders can get their fill. One option is to just feed large foods. Foods that are too big to be eaten whole require the aggressive feeders to grab a piece of food in their mouth and swim around with it as it softens, taking small bites as it does. While they are occupied with this food the more skittish feeders can get their share without worrying about being harassed by the aggressive feeders.

Homemade Foods:

Trying to develop a homemade diet for fish is very complicated. Unless the aquarist is also an animal nutritionist this should not be attempted. Trying to develop a fish diet without the proper background and education for it is simply guesswork. No fish’s health should be left to just guesswork. Let the professionals on staff at companies that make food for a living do it right for you.

You Get What You Pay For:

In general the old adage holds true, you get what you pay for. The most expensive foods are expensive because the company needs to pay for both high quality ingredients and research to develop the proper diet. Higher quality ingredients cost more, plain and simple. That cost will be part of the cost of the food. The best companies will put a lot of effort into making sure they provide the best diets possible. This also costs money. It is very cheap for a company to simply toss in fillers that are not digestible, cheap ingredients (usually the by-products of processing foods for human consumption), or simply copy some other company’s formula. This can seem like a good deal to the consumer standing in the aisle of their local pet store, but this low quality diet can cause problems long term.