Feeding aquarium fish: what, how much and how often

"How much should I feed my fish?" is a question most people answer with a pinch and a feeling. According to veterinary sources, that feeling is the most common reason aquarium fish get sick or die: not hunger, but too much food that the fish either leave uneaten or eat and excrete into the water as ammonia. Feeding is the only thing you do to an aquarium every day, which is exactly why it is the most common mistake.

This guide brings together what the MSD Veterinary Manual, the RSPCA (UK and Australia), the University of Florida and a published study on meal frequency in juvenile fish say about feeding aquarium fish: how much and how often, which food suits which fish, how many days fish can go without food, how to store food and where the sources disagree. It does not replace advice from a veterinarian who treats fish.

How much food do aquarium fish need

Less than you would think. The rule both RSPCAs give is simple: offer only as much as the fish eat within a few minutes. The UK RSPCA says 2 to 5 minutes, RSPCA Australia under 2 minutes. If food is still floating on the surface or lying on the bottom after that, you gave too much, so give less next time.

Veterinary sources calculate differently, by body weight, and they do not fully agree with each other. The pet-owner edition of the MSD Veterinary Manual gives a maintenance ration of 1 to 2% of the fish's body weight per day and a growth ration of 3 to 5%, while the professional edition of the same manual says adult fish should ideally be fed 3% of body weight daily and fry up to 5%. For a home aquarium the difference does not matter, because nobody weighs their fish; but it tells you something important: these are percentages of body weight, not pinches. A fish weighing 2 grams needs 0.02 to 0.06 grams of food a day, which is a few flakes, not a full cap.

A pinch is genuinely hard to judge, because flakes are very light and granules are heavy. MSD warns that different types of feed have dramatically different weights; a pinch of granules can be five times more food than a pinch of flakes. That is why the minutes rule is more useful at home than the percentage rule: you watch the fish, not the scales.

How often to feed aquarium fish: once or several times a day

One to three times a day, in small meals. The sources differ on the number, not on the principle. The UK RSPCA recommends two or three small feeds a day rather than one large one and gives the example of 7 am, 12 pm and 6 pm; fish learn the routine and will be waiting for you at those times. RSPCA Australia advises one feed a day, eaten in under two minutes. The MSD Veterinary Manual says most aquarium fish should be fed at least daily. The common thread: the total daily amount stays small, but it can be split into several meals.

For fry the answer is different. A study published in Data in Brief (2020) fed 30-day-old Siamese fighting fish and guppy juveniles one, two, three or four meals of micropellets a day for 60 days. In the fighting fish, growth in length was best at three meals a day, and survival was 70% compared with 50% on one meal; the guppies survived in every group. MSD supports this indirectly: fry and fingerlings can be fed up to 5% of body weight daily, more than adults. If you are raising young fish, feed them more often and in finer particles.

Large carnivores are the exception. According to MSD, fish that eat large meals (large cichlids, for example) may be fed once or twice a week. A beginner's tank usually has no such fish, so guppies, tetras, catfish and bettas get fed daily.

Which food is right: flakes, granules, tablets, frozen or live

The staple should be a good commercial dry food, flakes, granules or pellets, because according to MSD it is usually formulated to contain the right mix of nutrients when fed properly. Everything else (frozen, live, vegetables) is a supplement, not the base. MSD recommends one primary food source supplemented with others that supply the missing nutrients; the RSPCA says supplementing flakes matters both for balanced nutrition and for enrichment.

When choosing the form, go by where the fish naturally feeds. MSD makes this the first rule: fish that feed at the bottom should not be given flakes that float on the surface. Surface fish need food that floats, bottom fish need food that sinks quickly. Particle size has to fit the mouth: flakes can be too big for tiny fish (crush them) and too small for large cichlids, which cannot eat enough of them and foul the tank instead.

What about protein? In its professional edition, MSD says frankly that exact nutritional requirements are known for only a handful of fish species and that it is dangerous to assume food for one species suits a similar but unrelated one. What can be established is at least whether a fish is herbivorous, omnivorous or carnivorous: herbivores need more fibre, carnivores more protein and fat, and many fish in general have a higher protein requirement than other vertebrates. In practice this means buying food made for your group of fish (tropical, goldfish, catfish, cichlid) rather than a universal one. The RSPCA puts it with an example: do not give a goldfish tropical flakes, because they contain different levels of nutrients.

What individual aquarium fish eat

Almost all beginner species are omnivores and take good flakes or granules; the differences lie in where they eat and what they need on top of the staple. The table links the most common species from our aquarium fish directory with what the sources say about their diet.

For more on goldfish, how much they eat, why round-bodied varieties should not gulp air at the surface and what to give them instead of flakes, see our goldfish guide. Which species suit a beginner at all is the subject of 10 best aquarium fish for beginners.

Why overfeeding is the most common mistake

Because it does not show immediately and because fish always "beg". RSPCA Australia states that overfeeding is the most common problem with all fish: it leads to obesity and other health issues, and uneaten food breaks down and contaminates the aquarium. According to MSD, improper nutrition is one of the most common contributors to illness and death in aquarium fish, and parasites such as Tetrahymena and Uronema thrive precisely in water with a high level of organic matter, meaning faeces and uneaten food.

The key fact most owners do not know comes from the University of Florida (UF IFAS): most of the ammonia in an aquarium comes not from leftover food but from digesting the food the fish have eaten. The more a fish eats, the more ammonia it excretes through its gills. Leftovers on the bottom are only an addition. So it does not help that the fish "eat everything": if they eat too much, the ammonia is produced anyway. UF IFAS explicitly names overfeeding as a major cause of high ammonia. What ammonia does to fish and how to get rid of it is explained in our article on the nitrogen cycle.

Fish are more susceptible to poor water quality than to a lack of food, which is why, according to the RSPCA, it is better to underfeed than to overfeed.

How long can fish go without food: weekends and holidays

A few days without trouble; a week is a limit the sources are silent on. The University of Florida writes that 1 to 3 days without feed will not typically have a negative effect on healthy fish (fry are the exception) and that even a starved fish produces some ammonia; MSD adds that large carnivores eat only once or twice a week. For a long weekend, then, healthy adult fish need nothing special. It is better for them to go a few days without food than to be given a double portion before you leave.

For a longer holiday the UK RSPCA advises differently: tropical fish need daily feeding, so a responsible person should check the fish and the equipment every day. Leave them written instructions on how to feed and how to avoid overfeeding, and prepare the meals in advance in measured portions. The alternative is an automatic feeder that releases a measured amount each day; it too needs checking daily. None of our sources gives a number of days after which fish without food necessarily suffer, so beyond three days do not guess: arrange feeding.

Holiday feeding blocks ("weekend" or "14-day" food in a slowly dissolving block) are not recommended by the RSPCA: they release a lot of food into the water very quickly and most of it may rot. You may come home to ammonia rather than hungry fish.

How to store fish food and when to throw it away

Cool, dry, sealed and not for too long. MSD warns that improper storage of fish food is a common cause of nutritional imbalance: fish feeds are high in protein and oil, which deteriorate rapidly, and high humidity and temperature speed up the breakdown of nutrients, mould growth and rancidity. Fish need vitamin C from their food, and according to MSD a deficiency causes so-called "broken back disease", a severe curvature of the spine.

The sources estimate shelf life differently, so we give both. MSD (both editions) says an opened tub of dry food kept at room temperature should be used or discarded within 2 months, and that it is better to buy smaller quantities more often than one large container used for many months; dry food kept in the freezer lasts 6 months, and commercial frozen foods (brine shrimp, bloodworms) 1 year. RSPCA Australia is less strict: store food in a cool place and discard anything not used within one year of opening. If in doubt, follow the stricter figure; the vitamins in an opened tub do not wait for you.

Can I give my fish human food

Some vegetables yes, meat and fat no. The RSPCA writes that goldfish love chopped vegetables such as peas and spinach, and that tropical fish, depending on the species, will also take crumbled boiled egg yolk, a lettuce leaf or thawed fish; certain plecos need vegetables such as potato and a piece of wood to graze on. MSD recommends fresh courgette: diced, lightly cooked and stored in the freezer, a few times a week as a source of B and C vitamins. RSPCA Australia suggests scalded peas for goldfish.

What not to give: RSPCA Australia states explicitly that mammalian fats (beef heart, meat, offal) are poorly digested by fish and should not be fed. Bread, the food people most often throw to fish, is not mentioned as suitable by any of our sources, so stick to vegetables and fish food. Remove any leftover vegetables the same day, or they become the same problem as too many flakes.

Three myths about feeding fish

"Fish cannot overeat; they just eat while there is food." The second half is true, the first is not. RSPCA Australia calls overfeeding the most common problem with all fish and links it to obesity, while MSD lists fatty liver (hepatic lipidosis) among the common problems of captive fish and names too much fat and carbohydrate in the diet among its causes. A fish eating does not mean a fish that is hungry.

"Fish gasping at the surface are hungry." No. In its fish health guide the RSPCA writes that a fish continuously gulping and gasping at the surface is a sign of oxygen starvation or poor water quality; the exception is labyrinth fish such as gouramis, which breathe air normally. The answer is a water test and aeration, not more food.

"If the fish eat everything, there is no ammonia." No. According to UF IFAS, the primary source of ammonia in an aquarium is the digestion of the food eaten, not the leftovers. More food means more ammonia, even if the bottom stays clean.

Frequently asked questions

How much food should I give my fish?

As much as they eat in 2 to 5 minutes (RSPCA), or under 2 minutes according to RSPCA Australia. If food is still floating or lying on the bottom after that, it was too much. The veterinary measure is 1–3% of the fish's body weight per day, which for a small fish is only a few flakes.

How many times a day should I feed aquarium fish?

One to three times, in small meals. The RSPCA recommends 2–3 small feeds a day, RSPCA Australia one, and MSD at least one daily. Feed fry more often, and large carnivorous fish only once or twice a week.

How many days can fish go without food?

According to the University of Florida, 1 to 3 days without food does no harm to healthy adult fish; fry are the exception. For longer absences the RSPCA recommends a person who feeds daily or an automatic feeder. There is no reliable source for a longer limit, so do not guess beyond three days.

Are holiday feeding blocks a good solution?

The RSPCA does not recommend them: they release a lot of food into the water at once and most of it rots. For a holiday, use an automatic feeder or pre-measured portions given by another person.

Why are my fish not eating?

Most often because of the water: according to UF IFAS, fish under ammonia stress do not eat, and according to MSD lethargy and refusing food are typical signs of new tank syndrome. Test ammonia and nitrite and check the temperature before thinking of disease or the wrong food (flakes too big for small mouths, floating food for bottom feeders).

Can catfish live on the other fish's leftovers?

No. MSD states explicitly that expecting corydoras and plecos to survive on leftovers is not acceptable; they need their own sinking food. The RSPCA recommends feeding nocturnal catfish last thing at night, when the other fish are quiet.

Is live food better than dry food?

It is not complete. According to MSD, live food is a good source of protein and fat but can lack other nutrients, and wild-caught live food can introduce parasites. Fish fed only live food from the start often refuse anything else. Use it as a supplement to a good dry food, from a reputable source.

How long does opened fish food keep?

According to MSD, 2 months at room temperature and 6 months in the freezer; commercial frozen food 1 year. RSPCA Australia allows 1 year after opening. Buy smaller tubs and store them dry, cool and sealed.

Can I give fish bread or meat?

Not meat or mammalian fats: RSPCA Australia writes that fish digest them poorly. Bread is not listed as suitable food by any of our sources. Suitable supplements are blanched peas, courgette and spinach (RSPCA, MSD) and, for tropical fish, frozen daphnia or brine shrimp.

In closing

If you remember one thing: feed your fish with your eyes, not your hand. A small meal eaten within a few minutes, one to three times a day; sinking food for the fish that eat at the bottom; vegetables for those that need them; an opened tub used up within two months. And when your fish look hungry, test the water first. How to set up an aquarium in which feeding is safe in the first place is explained in our guide How to set up your first aquarium.