Are Most Fish Omnivorous? — New Life Spectrum®: Fish Food

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Are Most Fish Omnivorous?

Let’s take a common misunderstanding about fish nutrition — the idea that nominal herbivores benefit from a veggie-based diet alone. We’ll be examining whether the vast majority of aquarium fish are in fact omnivores…

To the marine aquarium enthusiast, Atlantic Blue Tangs (Acanturus coeruleus) are considered herbivorous — and they are indeed grazers, with lips and dentition designed for snipping off the tips and branches of algae.

However, the study “Captive Nutritional Management of Herbivorous Reef Fish” from the University of Florida (by Dr. Ruth Francis-Floyd and Chris Tilghman) has confirmed they need more than just algae in captivity. The feed trial used Atlantic Blue Tang, divided into three groups, as test subjects. The first group was fed washed seaweed (ulva spp.). The second was fed commercial food designed for herbivores. The third group was fed another all-purpose commercial diet (i.e. marine protein was in the formula). The first and second group suffered high mortality rates of about 80% — with the surviving fish showing clinical signs of malnourishment like emaciation. The third group had only an approximately 30% mortality rate and showed a 400% weight gain!

Information on this study was made available in a lecture on November 29, 2001, at the Marine Ornamentals International Conference held in Lake Buena Vista, Florida. Yet outside that conference room, the results were never made public, due to pressure to avoid controversy within the aquarium feed industry.

Most fish have specialized feeding methods and ingest a certain type of food matter more than others. However, in the wild almost all are opportunistic feeders. Even the most specialized, like Atlantic Blue Tangs, ingest a certain amount of nutrients from other sources.

Herbivores have been observed eating invertebrates. They also receive protein via organisms within the plant matter they ingest. Carnivorous fish get plant-based nutrients from the contents of their prey’s digestive tract (which is gut-loaded with various organics, including phytoplankton and zooplankton).

Beyond fish, other common aquarium dwellers like crustaceans are scavengers, and thus opportunistic omnivores. Filter-feeders likewise consume both phytoplankton and animal zooplankton.

For freshwater examples, we need look no further than the fish in the African lakes of Malawi and Tanganyika. These are some of the most specialized feeders found on the planet — yet they too are opportunistic feeders. Algae does dominate the stomach contents of certain species of African cichlids classified as herbivores. However, protein-rich organisms in significant quantities are also present. Breeders know that the foods that make these fish grow are insect nymphs and larvae, crustaceans, snails, mites, micro-organisms, and zooplankton — not vegetable matter.

Feeding and the Omnivorous Aquarium: The reality that most tropical fish are omnivorous — with herbivorous or carnivorous specialization — has some clear implications when feeding your companions. First, any commercial herbivore diet should include sources of protein, and any diet aimed at carnivorous fish should contain aquatic plant matter.

Wild herbivores must graze at least 12 hours daily to ingest enough nutrients. During grazing, they discharge waste almost constantly. In the artificial conditions of an aquarium, these fish don’t have unlimited food to graze upon. Even if you could feed them every hour, you couldn’t maintain the needed water quality due to fish waste. An aquarium isn’t even a fraction of a drop compared to the water volume of the ocean or a large lake. In order for nominally herbivorous fish to thrive in an aquarium, a higher nutrient density solution is required.

It’s helpful to think of an aquarium as an environment for your fish that is like a space station habitat would be for humans. In such a ‘contained’ environment with limited space and resources, the food you consume should be as little in mass, and as nutrient-dense as possible to reduce waste products in a closed environment and get the most benefit out of each one of your limited daily meals. Accordingly, meals prepared for astronauts are usually focused on packing as much nutrition into each morsel as possible.

Hobbyists may believe feeding Surgeonfish a diet of algae (or commercial foods that are purely plant based) is more ‘natural’ than a pellet or flake food. That couldn’t be further from the truth. First hand experience with attempts at wholly plant based diets have failed miserably — and the results from the study performed at the University of Florida bear these conclusions out.

Carnivorous fish likewise have a much different life in the aquarium than in the wild. Outside captivity, their prey is part of the natural food chain, and is gut-loaded with nutrients derived from their environment — but which the carnivores don’t eat directly. In particular, plant matter. Frozen silversides found in your local pet shop don’t provide the same, needed nutrients as wild prey. The reality is carnivores don’t just eat meat, any more than herbivores just eat algae.

In the Next Article we’ll be talking about Nutrition Basics



Tilghman, G.C, R. Klinger-Bowen and R. Francis-Floyd. 2001. Feeding electivity indices in surgeonfish (Acanthuridae) of the Florida Keys. Aquarium Sciences and Conservation 3:215-223

E. L. Ferreira, J. E. A. Gonçalves (2006) Community structure and diet of roving herbivorous reef fishes in the Abrolhos Archipelago, south-western Atlantic Journal of Fish Biology 69 (5), 1533-1551.

Ferguson, H.W. et al. Gastritis in Lake Tanganyika cichlids (Tropheus duboisi). In Vet Rec., 1985, 116, 687-689

Ako H., and Tamaru C.S. (1999) Are Feeds for food Fish Practical for Aquarium fish? Intl. Aqua Feeds 2, 30-36.

Fox, D. Biochromy. Natural Coloration of Living things. 1979. University of California Press, Ltd. London, England.

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