When talking about fish nutrition for freshwater and marine tropical fish, we’re really talking about a lot of variables and a lot of unknowns.
Only the nutritional requirements of only about a dozen fishes have been studied in detail (Fish Physiology: Hoar & Randall, Academic Press, NY) and these have all been fishes important to aquaculture.
Many commercial foods for tropical fishes are based on these studies of aquaculture species. Many others are based on duplicating (in frozen or freeze-dried form) at least some of the foods tropical fishes are known to eat in the wild and/or on formulations approximating the chemical make-up of these natural foods.
The problem is, many fishes eat a variety of foods to obtain their nutritional requirements. Some of these foods are esoteric and not widely available in substitute form. A perfect example of this is the rotting driftwood diet of certain Panaque and other Loricariid catfishes.
Many of the natural foods of fishes have not been analyzed for nutritional compounds nor have there been any detailed studies of gut contents of collected fish to determine exactly what their natural foods are.
When specimens are collected for science, their stomach contents are usually identified, but that only shows what the fish has eaten recently and does not give a thorough picture of all the foods it naturally eats.
So, as an aquarist, what can you do to be sure your fishes are getting the proper diet?
The answer, in general, is simple. Feed a variety of foods: live, frozen and prepared (flake or pellet). There are a few other considerations to bear in mind.
First, KNOW your fish.
The only way you can determine if your fish have special dietary needs is to learn all you can about them.
This means asking questions on online forums, attending local aquarium society meetings and talking to members who have interests similar to yours; reading all that you can: aquarium magazines, newsletters, society exchange publications, books; visiting public aquariums and, if possible, talking to the aquarists there.
Second, READ Labels.
Fish food sold in pet shops is labeled according to protein, mineral, carbohydrate, ash and water content. Since “ash” is non-digestible matter, the lowest ash content is the best.
The same with water.
The water in fish food (unless frozen food) is a product of the way the food is manufactured. The less water, the better. Water in frozen foods is a combination of the water present in the tissues of the frozen animal and the water used to prepare it for freezing.
Food coloring is also listed on labels. This is placed in flake food mostly, to make it more eye appealing to the user, not to the fish, which find their food by scent more often than by color.
Third, UNDERSTAND how and why your fishes need protein
Fast, active, fish such as many tetras, danios, damsels, etc., need more protein than do more sedentary fish such as many catfish, hawkfish, etc.
Protein is burned up as calories in exercise. Fish also need more protein for egg production. If you want to spawn fish, feed them a high-protein diet.
Temperature plays a large part in how fast a fish metabolizes protein. Fish kept at higher temperatures need more protein than do the same fish at lower temperatures. Fishes normally kept at lower temperatures may need more protein if they are very active, e.g. trout.
You can see why there is really no hard and fast rule here. The number of variables is considerable.
Finally on this subject, strictly pisciverous fishes have evolved metabolisms that are more dependent upon animal protein than have omnivorous or herbivorous fishes. That is not to say that plant protein is not important to the omnivores or herbivores. But IT IS A DIFFERENT KIND OF PROTEIN and must be supplied in the form of plant protein. There are a number of different plant flake foods that supply this. One of the best sources of plant protein is spirulina, and spirulina flakes are available for feeding fishes.
Fourth, REALIZE that there are important differences in the other ingredients our fish need to eat
Chief among these are the lipids (fatty acids).
Sometimes you’ll see these listed on fish food containers as “fat”. That’s not much help. There are several dozen lipids; some more, some less, vital to fish health and proper nutrition.
In general, follow this rule: NEVER feed fresh water fish to marine animals and vice versa. OK, once in a while won’t hurt but the fatty acids in fresh water animals and marine animals differ and a prolonged diet will cause fatty acid deficiency.
Fatty acid deficiencies result in reduced growth, higher percentages of muscle tissue water, liver degeneration, higher susceptibility to bacterial infection, decrease of hemoglobin in the blood cells, and possible shock (Nutrition and Feeding of Fish, Lovell, AVI Books).
If, for instance, you feed pisciverous marine fishes a diet of strictly feeder goldfish, you will shorten their lives and prevent their attaining full growth and best condition. If you feed pisciverous (fish-eating) freshwater fish a diet of, say, frozen smelts or prawns, you’ll do the same harm.
These remarks apply to freshwater and marine invertebrates as well and for the same reasons. For obvious reasons, I do NOT feel that the old hobbyist standby, beef heart, is of any lasting benefit as a fish food. It does promote growth but it also increases the chance of liver malfunction or malformation, among other problems.
A Few Words About Vitamins
Essentially, all of the 15 vitamins are needed by fish though not all species need all 15 vitamins. There is still much research needed. Two of the most important vitamins for all species are probably Vitamins C and D.
The vitamin needs of fishes are poorly understood.
We do know that fish, along with primates (that includes us), monkeys, fruit bats and guinea pigs, cannot biosynthesize Vitamin C from the proteins and amino acids in the prepared foods they eat. They don’t have the enzyme system necessary to convert L-gluconic acid to ascorbic acid. They must ingest Vitamin C directly, either from ascorbic acid occurring naturally in their food or from ascorbic acid added to prepared foods.
Vitamin C (ascorbic acid) in fishes is directly related to the formation of procollagen fibers necessary for the biosynthesis of collagen and cartilage, tissue formation and repair and calcification of bone.
Wild and pond-raised fish obtain Vitamin C from the insects, vegetation and other fish that they eat. Tank-raised fish depend upon ascorbic acid added to commercial feeds. This has always been a problem because Vitamin C decomposes rapidly in contact with water. This is because oxygenated water oxidizes ascorbic acid to form dehydro ascorbic acid. Increased temperature accelerates the process.
Water, in the form of food processing, humidity, and, of course, the water the fish live in, decomposes the Vitamin C added to commercial fish foods. Production and storage losses of Vitamin C will vary according to the process used to make the food. All commercial fish food is manufactured by some process involving oxygenated water and heat.
Foods with high moisture content lose Vitamin C more rapidly than those with lower moisture content though storage conditions also play a large role.
In general, pelletized floating foods are processed at higher temperatures and so lose more Vitamin C during the manufacturing process. A quick label check of some popular flake foods shows moisture content to range from 6-8%. Some pelletized foods range from 8-9%.
Fish food manufacturers have tried to overcome this problem for years. Some have added 10 to 20 times the recommended level of Vitamin C to their product to compensate for the loss in manufacturing and to add storage life to their product. This is an expensive step but fish deprived of Vitamin C may be anorexic or develop skeletal abnormalities. They may also be slow to heal from otherwise superficial wounds.
Until the problem is solved at the manufacturing end the following precautions will help:
- Store all fish food in dry, air-tight, containers.
- Avoid reaching into containers of manufactured food with wet hands.
- Buy manufactured foods in small quantities from dealers you know have a rapid turnover.
- Avoid buying food from places where it sits on a shelf for a long time (i.e. a supermarket).
- Do not buy foods from manufacturers who crank it out in multi-ton lots and warehouse it indefinitely. Generally, these are the same brands sold in supermarkets and discount stores.
- Do not store fish food in a damp environment.
Vitamin D is found in nature in two forms: ergocalciferol (D2) and cholecalciferol (D3). According to Lovell, ultraviolet radiation of two pro-vitamins, ergosterol (found in plants) and 7-dehydrocholesterol (in skin of animals) produce D2 and D3 respectively.
Most land animals, except chickens, use D2 and D3 interchangeably. Fish use D2 poorly or not at all. The role of Vitamin D in fish diet is little researched and poorly understood but, since fish get little natural ultraviolet energy as that form of sunlight only penetrates water for a few inches, it must be assumed that Vitamin D3 forms an important part of their diet. Fish foods containing fish oils are a good source of Vitamin D3.
Since freshwater fishes do not drink water, adding liquid vitamins to a tank is useless. These may benefit saltwater fishes which do drink water but it is impossible to accurately figure dosages.
If you must substitute vitamin supplements for proper foods, then add the vitamins to the food you’re feeding. A caution here. Use vitamins developed especially for FISH. Human-type vitamins contain the wrong kind of Vitamin D.
Gelatin-based foods are the easiest to add vitamins to.
Another way to add vitamins to fish food is to add vitamins to the water live (adult or nauplii) brine shrimp are in. The shrimp will ingest the vitamins and the fish in turn will ingest both shrimp and vitamins. (This is also one of the best ways of medicating a particular fish rather than medicating the water or the entire tank of fish).
Of course, with the proper mix of live and prepared foods, fed appropriately (freshwater foods to freshwater fishes, saltwater foods to saltwater fishes) the addition of vitamins should be superfluous.
Know any special dietary requirements of your fish. The best ways to do this is to join an aquarium society, join a specialized society (such as the American Cichlid Association, Rainbowfish Study Group, etc.); talk to hobbyists with similar interests at society meetings, shows.
Read everything you can get your hands on pertaining to the fish you keep or want to keep; and visit public aquariums and, if possible, talk to the professional aquarists there.
Read labels on commercial fish food containers as carefully as you would on containers of food you’d eat yourself.
Keep flake, freeze-dried and pelletized fish foods dry and purchase them in small quantities. Keep wet hands out of food containers. Avoid feeding freshwater foods including live feeders to marine fish and vice versa. (I doubt many people will purchase Dascyllus spp. to feed to their Oscars but flake foods for marine fishes should NOT be fed to freshwater fishes due to different fatty acids used in them.
Similarly, a diet of frozen prawns or other frozen and freeze-dried marine foods is not in the best interests of freshwater fishes.) Finally, feed your fishes a variety of foods, both prepared and live. This way you’ll insure that they are getting all the essential vitamins, minerals and protein that they need.