Treats and Supplementary Foods for Aquarium Fish: What They Add and What They Risk

Quick answer

Bloodworm, brine shrimp, daphnia and dried krill are not a balanced diet. They are supplements to a complete staple food, and the trouble starts when they quietly become the main meal.

Fish are ectotherms. Their energy demand is set by water temperature, not by appetite, and a fish will keep begging long after it has eaten everything it needs. That is foraging behaviour, not hunger. Every extra portion also lands in the water, where it becomes ammonia.

• Treat foods replace part of the ration for that day. They are never added on top of it.
• Rinse thawed frozen food in a net and never pour the pack liquid into the tank.
• Soak freeze-dried foods before feeding, because they expand after swallowing.
• High-protein foods such as bloodworm are genuinely dangerous for herbivorous cichlids.
• Live food collected from ponds or sold from communal tanks can import parasites and disease.

Decide in 60 seconds

Why extra food costs more than it looks

A fish's energy budget is driven by water temperature. Warmer water raises metabolic rate and appetite, cooler water lowers both. There is no fixed daily portion that holds across a season, and a portion that is right in summer is too much in an unheated tank in winter.

The second cost is invisible. Every particle of food that is not eaten, and every gram of waste produced by food that is eaten, is broken down into ammonia by bacteria in the tank. The biological filter converts ammonia to nitrite and nitrite to nitrate, but the bacterial population that does this is sized to the current load. Increase the feeding sharply and the filter takes days to catch up, during which ammonia and nitrite rise. Fish are damaged by both at levels far below anything you can see.

Rich foods add a third cost. Frozen foods are sold in a block of liquid that contains broken-down tissue and blood. Poured straight into the tank, that liquid is a dissolved organic load that goes to ammonia immediately without the fish ever eating it.

The fourth cost is nutritional balance. A complete staple food is formulated to provide protein, fat, vitamins and minerals in ratios appropriate for the species group it is sold for. A single-ingredient food is not, no matter how natural it seems. A tank fed mostly on bloodworm is a tank on an unbalanced diet, and the deficiencies show up slowly as poor colour, poor growth, fin problems and susceptibility to infection.

Measuring a portion you can defend

A dedicated scoop and a fixed measure remove the biggest variable in fishkeeping, which is the hand that does the feeding.

Feed a measured amount from a scoop rather than a pinch from the pot, and use the same scoop every time. Different people in a household give wildly different pinches, and multiple feeders is the most common cause of an overfed tank.

Watch the tank for a few minutes after feeding. The reference point is simple: food should be gone within a short, consistent window, and nothing should be sitting on the substrate afterwards other than the deliberate portion for bottom dwellers.

Judge condition from above rather than from the side. A fish viewed from the side always looks reasonable; viewed from above, the shoulders and the belly tell you whether it is lean, correct or fat. Learn the outline of your own fish when they are in good condition and check it monthly.

Bottom-dwelling fish need food that reaches them. Sinking wafers dropped at a fixed spot, ideally after lights out for nocturnal species, prevent the situation where a catfish slowly starves in a tank that is being overfed at the surface.

What each supplementary food actually does

Frozen bloodworm.

Chironomid midge larvae. High in protein, very palatable, useful for conditioning fish and for tempting a fish that has gone off food. Not a complete food and not appropriate as a staple. Unsuitable for herbivorous species. Handle with a scoop rather than fingers, because sensitisation and allergic reactions in aquarists who handle it frequently are well recognised.

Brine shrimp.

Newly hatched brine shrimp are a genuinely valuable first food for fry. Adult brine shrimp are mostly shell and water, and their nutritional contribution is modest unless they have been enriched before feeding. Useful for stimulating feeding behaviour rather than for nutrition.

Daphnia.

Small crustaceans with a chitinous shell that adds bulk and moves things along the gut. Frequently used as the food to offer after a period of heavy feeding, and a sensible part of a rotation.

Mysis shrimp and krill.

Larger, richer, higher in fats and in the pigments that support colour. Good conditioning foods for mid-size and larger fish, easy to overdo.

Tubifex and blackworms.

Tubifex has a poor reputation for good reason: it is a sediment worm and material harvested from contaminated water has historically carried bacterial problems. Cultured blackworms from a reliable supplier are a different proposition. Neither is necessary, and if you use them, source matters more than anything else.

Live foods generally.

Live food triggers natural hunting behaviour, which is real enrichment, and it will restart the appetite of a fish that has stopped eating. The cost is biosecurity. Anything collected from a pond can carry parasites, and anything sold out of a shop tank shares that tank's water. Cultured live food from a closed system, or hatched at home, avoids most of this.

Vegetables.

Blanched courgette, deshelled peas and blanched spinach suit herbivorous and omnivorous fish and are genuinely useful for grazing species that spend their day picking at surfaces. Remove what is not eaten within a day. Wood-rasping catfish need a piece of suitable wood in the tank as well, because the lignin is part of their digestion rather than a decoration.

Feeder fish.

Not recommended. They are nutritionally poor, they are a well-known route for introducing disease into a tank, and some species commonly sold as feeders contain thiaminase, an enzyme that destroys vitamin B1 in the predator that eats them. Chronic thiamine deficiency produces neurological signs and is entirely avoidable by feeding prepared foods designed for predatory fish.

Condition is read from above, not from the side. Learn your own fish's healthy outline and check it monthly.

How to feed treats without the side effects

Thaw and rinse frozen food.

Break off a portion, thaw it in a small container of tank water, then tip it into a fine net and let the liquid drain away. Feed the solids. The pack liquid is the part that pollutes.

Never refreeze.

Thawed portions are discarded, not returned to the freezer. Break the block into portions once, at the start, so you never thaw more than a single feed.

Soak freeze-dried foods.

Freeze-dried krill, tubifex and bloodworm expand as they rehydrate. A fish that gulps them dry at the surface swallows air alongside them and then rehydrates the food inside its gut. In fancy goldfish and bettas, both of which already have crowded internal anatomy, this contributes to buoyancy problems. Soak in tank water for a minute or two first.

Replace, do not add.

On a day when treats are given, reduce or skip the staple. The commonest way to overfeed a tank is to keep the routine intact and add something interesting on top of it.

Rotate rather than repeat.

A staple plus two or three different supplementary foods across a week gives better coverage than the same treat every day, and it also prevents fish becoming fixated on one item and refusing the balanced food.

Feed to the species, not to the enthusiasm.

Herbivores get plant material. Carnivores get meaty foods. Omnivores get both. The fish that begs hardest is not the fish that needs the food.

Match the season in a pond.

Digestion in pond fish shuts down as water cools, and food fed below the working range for the fish sits in the gut and decomposes. Use a thermometer in the water rather than judging by air temperature, follow the temperature range printed on the food, and switch to a cool-water formulation before the fish stop feeding, not after.

What never to do

Do not pour the liquid from a frozen food pack into the tank.

Do not feed live food collected from a natural pond into a tank you care about.

Do not feed bloodworm or other rich protein foods to herbivorous rift lake cichlids, goldfish kept as staple, or any species whose natural diet is plant material.

Do not add treats on top of the normal ration and assume a fasting day will fix it.

Do not use a feeding block or holiday feeder as a routine convenience. They dissolve on a schedule that has nothing to do with what the fish eat, and the residue goes straight to the water.

Do not feed a pond in cold water because the fish came to the surface. They come to the surface out of habit, not digestive capacity.

Do not handle bloodworm repeatedly with bare hands if you notice itching, sneezing or a rash after feeding.

When to get help

Seek advice from a vet who sees fish, or an experienced specialist retailer, when a fish stops eating for more than a day or two, when the abdomen swells, when faeces become white and stringy, or when a fish's outline changes without any change in feeding.

Act immediately on a swollen, off-colour rift lake cichlid, on any fish showing neurological signs such as spinning or loss of balance, and on an ammonia or nitrite reading above zero after a feeding change.

A fish in condition, in stable water, on a staple diet with occasional variety, is the whole objective.

Bring the exact staple food and every supplementary food you use, how often each is given, the measured portion, recent water test results for ammonia, nitrite, nitrate and pH, the water temperature, the tank volume and stocking, and monthly overhead photographs. Most feeding problems in fishkeeping are diagnosed from that list rather than from the fish.