Diet is the most important aspect of animal care. In aquariums this 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.

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 has been discusses in the article ‘Water Changes and Stocking an Aquarium’. Diet is more complicated however. 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 of the fish. In general more carnivorous fish have shorter, simpler digestive systems. More herbivorous fish 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 fishkeepers. 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 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. 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. 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 fatty foods 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. The same instinct that drives us to fill up on fried food followed by ice cream helped us get through the leaner times our ancestors faced.

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 unnatural high drive for such foods can lead to poor overall diet when they are offered 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 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 do 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 simply 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 type of ingredients the average aquarist things 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 addition to this, the company may simply use parts of that ingredients 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 mbunas). Once it passes through the fish and discharged into the water column it can be broken down into ammonia and eventually nitrate. This can raise the nitrate concentration in the tank. So high quality food is not just good for the fish, but also for the health of the whole tank.