Overfeeding Fish

In this age of controversy and discord, it’s difficult to find any issue, whether in politics, child-rearing, or even fishkeeping, in which “conventional wisdom” goes unchallenged. It’s refreshing to find that there’s at least one area of our hobby that all of us, from expert to hobbyist to novice, can agree upon: it’s important not to overfeed your fish.

But even that sage advice falls short, as sage advice sometimes does, when it comes to addressing the questions of real life experiences. What is overfeeding? What problems does it cause? And how do I know if I’m doing it?

Overfeeding is simply putting more food in the aquarium than the fish can use. Uneaten food is the biggest danger, but even surplus food that has been eaten can wind up back in the tank as excess fish waste. It’s a common misconception that overfeeding fish causes some sort of gastro-intestinal damage that leads to their untimely demise. While it’s certainly plausible that a fish, for example a particularly gluttonous goldfish, might overeat enough to develop a long-term health risk, the greatest risks of overfeeding are the result of the uneaten food left to rot in the aquarium. Uneaten food decomposes quickly, releasing an abundance of organic and inorganic compounds that wreak havoc on the aquarium’s chemistry. There are several common symptoms of overfed fish tanks:

Cloudy water is almost always associated with something organic rotting in the tank, and most aquarists confronted by this problem need look no further for the source than that can of flake food on the shelf. In fact, a good test for the cause of cloudy water is simply to quit feeding the fish altogether for a couple of days (it’s ok, very few fish are incapable of surviving a fast for a few days). If the water clears up, feeding habits (including size, timing and type of meal) need to be adjusted.

Algae growth takes a little longer to develop, but is probably even more common than cloudiness in overfed aquariums. Many algae, especially the slimy sheets of blue-green or red algae- impersonators (technically bacteria rather than true algae), thrive when there is an abundance of dissolved organics, nitrates and phosphates in the water. All of these are produced in quantity as uneaten food decays. A little algae growth is normal and fairly unavoidable, but if the tank needs scrubbing every few days, it’s likely there is an excess of food or fish waste accumulating.

Little patches of fungus or mold will occasionally be seen growing on the gravel and plants. This is directly attributable to uneaten food. Where a flake or grain of fish food falls to the bottom or sticks to a plant, a little patch of white cottony stuff will sprout up, grow for a few days, and then disappear when its food source is used up. It may of course be replaced by others if overfeeding continues.

Tiny, hair thin worms will sometimes appear on the walls of an aquarium. These are almost always found in tanks of either young fry or large, pellet-munching fish like oscars, as they are generally eaten in tanks of more typically-sized community fish. They feed directly upon excess fry food or the “dust” that blows out of the gills of a large predator fish each time he gulps down a dry pellet or other soft food.

Low dissolved oxygen, high nitrates, declining pH, and other chemical problems are also common in the overfed aquarium. Many of the decay processes that “eat” uneaten food are aerobic – which means they utilize oxygen to break down the organic molecules. This is a good thing, because anaerobic decay produces some really disgusting compounds, like methane and hydrogen sulfide, that the aquarist would just as soon do without. There is still a price to pay, however, as the increased demand for oxygen by the decay processes mean less oxygen available to the fish. In addition, proteins in uneaten fish food are eventually broken down into nitrates and leftover hydrogen ions – just as fish waste is. The poor fish in the overfed tank have to deal with the pollution caused by the food without receiving the benefits of having eaten it!

How to avoid overfeeding

Time your feedings. Fish in nature rarely get the opportunity to sit down to a leisurely meal. Perhaps that’s why most fish, when presented with a shower of flake food, gobble it up like there’srainbows no tomorrow, then suddenly stop eating when they can’t possibly squeeze in another bite. The time involved to finish dinner will vary from fish to fish; some of the smaller, eager-eaters like danios may get their fill in ten or twenty seconds, while the bottom-feeding catfish may need several minutes to search out a satisfactory meal. Watch your fish eat, and try to get a feel for how long and how much they actually eat when you feed them – and then adjust the size of the feedings accordingly. Keep in mind that any food left after five minutes will likely never be eaten, and only serves to foul the water. Repeat this time test occasionally, as differences in temperature and the number and sizes of fish, will affect the overall tank “appetite.”

Feed often, but sparingly. Some aquarists seem amazed that they have overfeeding-type symptoms, but only feed their fish “every other day.” For most of us, overfeeding has little to do with feeding too often, but much to do with feeding too much at a time. In nature, most fish generally don’t eat in “meals,” but graze or nibble all day as opportunities present themselves. Their bodies are designed to deal with food in just that manner: a little at a time, but very frequently. If these fish are fed only occasionally, but in massive quantities, they could well suffer both malnutrition and the resulting poor water quality from overfeeding. You can keep a healthy tank while feeding two, three or four times a day, but the key is to keep the size of those feedings under control. (The exceptions to this rule are some of the larger, predatory fish that may indeed go for long stretches without finding food and then gulp down a meal half their size.)

Feed the fish, not the tank. It stands to reason that a six small fish should get the same size feeding whether they’re in a 10 gallon tank or a 100, but we have a natural tendancy to gauge our fish’s meal size by the capacity of their living quarters, rather than that of their stomachs. The owner of six baby discus fish in a 55 gallon tank may not think twice about the possibility of overfeeding, but it’s remarkably easy to do, especially since those few fish will have a difficult time finding every bit of food that has scattered over such a large area.

Feed an appropriate food. Sometimes food goes uneaten not because the fish aren’t hungry, but because they can’t handle the food as presented. In the case of the tiny worms mentioned above, the pellet food might be too easily smashed to dust in the jaws of a large predator; a harder or a softer pellet may solve the problem. Flake foods may be too small for large-mouthed fish, or pellets may be too large for those with smaller mouths. Some foods may float while your fish are searching the bottom; others may sink right past a surface feeder. Some foods might be just too darn unnatural for a picky eater who last week was feasting upon the “real food” of his natural habitat . Once again, watching your fish eat will give you valuable information as to the acceptability of your offerings.

Scavengers help – at least a little. Many catfish and loaches wait until food settles to the tank bottom before beginning to feed, which makes them useful in cleaning up some of the scraps the other fish missed. This doesn’t really solve the problem of too much food, since the “perfect” amount of food is based on the total number of fish, including the scavengers. However, a few scavengers can help substantially when the problem is food that has become inaccessible to the other fish by falling beneath their feeding zone or getting trapped in the nooks and crannies of the tank’s decor.

No “junk” food, please. Who among us would save a couple of bucks on a can of fish food if we knew it would mean more water changes, or worse yet, lost fish? There are good quality fish foods available, and poor quality foods as well – and price alone may not be much of an indicator of which is which. But don’t keep feeding a food which isn’t working out well, and certainly don’t feed from that half bag you found in the attic from the tank you had two years ago. Good foods will be eaten readily – and completely – and will provide the nourishment that leads to healthy, growing and more colorful fish.

Try new foods carefully. Fish are creatures of habit, and will sometimes fail to recognize that the snowstorm of new, improved flakes falling all around them is food. That doesn’t mean the new food is no good, only that your fish are expecting something that looks, feels, smells and tastes just like the stuff they ate yesterday and the day before. They will usually adjust, especially if you feed a variety of foods anyway, but it pays to be extra cautious the first few times you try a new food. Feed very sparingly, and remove any food left uneaten after a few minutes. It may be necessary to discontiue feeding all the old favorite foods for a few days to further induce your fish to accept a change in diet.

Remove uneaten food immediately. This serves two functions: first, to reduce the amount of food left to decay in the tank; and second, to further train the aquarist as to how much and what his charges are consuming. Don’t count on your filtration system to do this job, unless you’re planning to clean it immediately. Uneaten food rots just as well in the filter as in the tank itself, with exactly the same consequences. Use a fine net or siphon and try to remove as much as is practical.

Finally, stick with it. The rewards of a cleaner tank and healthier fish are well worth the effort

This article originally appeared in

Aquarium Fish Magazine