Fish food labels: what the analysis tells you, and what changes with the fish

Quick answer

The label decides what the fish gets. Most aquarium fish eat one manufactured food for their whole life.

Most pet fish eat a single manufactured product for years, so an error in that product accumulates in a way it never would for an animal eating varied meals. The label is the only information you have about what is actually going in.

Two lines carry most of the value: the ingredient list, read in order, and the form of vitamin C. Almost everything else on the front of the pack is marketing.

• Crude protein is calculated from nitrogen and says nothing about how usable that protein is.
• Ingredients are listed by weight, so the first three tell you what you are really buying.
• Stabilised vitamin C survives manufacture and storage. Plain ascorbic acid mostly does not.
• Buoyancy and particle size matter more than the analysis if the fish cannot physically reach or swallow the food.
• An open tub loses value steadily. Buy the size you will finish, not the size that is cheapest per gram.

Decide in 60 seconds

What the analysis panel actually measures

Crude protein.
Determined by measuring nitrogen and multiplying by a fixed factor. That is why it is called crude. It counts nitrogen regardless of its source, so a food built on fish meal and a food built on plant protein and feather meal can show the same number and be nutritionally very different. Digestibility and amino acid balance are what actually matter, and neither appears on the panel.

Crude fat, sometimes shown as crude oil.
The energy fraction, and also where the long-chain omega-3 fatty acids sit for marine and cold-water species. Fat is the component that goes off, which is why a high-fat food demands better storage than a low-fat one.

Crude fibre.
Useful in the opposite direction depending on the species. For a herbivore, fibre is part of the diet and a low figure suggests the food is wrong for it. For a carnivore, a high figure usually means plant filler.

Crude ash.
The mineral residue. A moderate figure is normal and reflects bone, shell and added minerals. A very high figure suggests a lot of low-value mineral bulk.

Moisture.
Matters most when comparing across formats. Dry flake and dry pellet can be compared directly. A moist gel food or a frozen food cannot be compared to a dry one without converting both to a dry matter basis, which means dividing each nutrient by the proportion of the food that is not water. Skip that step and a frozen food will always look weaker than it is.

The whole panel is a set of legal minimums and maximums. It tells you what the product is guaranteed not to fall below, not what is in the batch in your hand.

The ingredient list, read properly

Ingredients are listed in descending order by weight before processing. The first three usually account for most of the product.

Named ingredients beat categories.
Herring meal, krill, black soldier fly larvae and spirulina tell you what you are feeding. Fish meal, fish and fish derivatives, and vegetable protein extracts do not, and they allow the manufacturer to change the source between batches.

Watch for splitting.
Wheat, wheat flour and wheat gluten listed as three separate entries can each sit below a protein source while together outweighing it. Add the related entries mentally before judging the order.

Some starch is unavoidable.
An extruded pellet needs a binder to hold together and to control whether it floats or sinks. The question is not whether starch is present but whether it is at the top of the list in a food for a carnivore.

Plant protein is not automatically filler.
For herbivorous and omnivorous species it is appropriate. For a strict carnivore, soy and pea protein are cheap nitrogen that the fish uses poorly, and the unused portion leaves as ammonia into your water.

Colour enhancers are real, and narrow.
Astaxanthin, spirulina, marigold extract and paprika supply carotenoid pigments that many fish deposit in their skin but cannot synthesise. In species that use them, a colour food genuinely restores colour. In species that do not, it does nothing. Synthetic dyes that colour the pellet rather than the fish are for the buyer, not the animal.

Antioxidants and preservatives.
Their job is to stop the fat oxidising. Whether the food uses synthetic antioxidants or natural tocopherols matters less than how you store the tub, because a natural antioxidant system generally gives a shorter useful life once opened.

The vitamin C line is the one to look for.
Ascorbic acid in its plain form is largely destroyed by the heat of extrusion and continues to degrade in storage. A stabilised form, usually listed as an ascorbyl phosphate, survives both. Vitamin C in fish supports collagen formation, healing and immune function, and a food that lost it in the factory has already lost it before you buy the tub.

Matching the food to the fish, which is where most errors happen

A goldfish has no true stomach, so it is built to take small amounts continuously rather than one large meal.

Mouth position tells you where the fish feeds.
An upturned mouth belongs to a surface feeder such as a hatchetfish, a guppy or a betta. A terminal mouth pointing straight forward belongs to a mid-water feeder. An underslung mouth on the underside of the head belongs to a bottom feeder such as a corydoras, a loach or a pleco.

This single piece of anatomy causes more starvation in community tanks than any nutritional shortfall. A floating flake never reaches a corydoras, and by the time it sinks the mid-water fish have eaten it. A tank with fish from more than one level needs more than one food format, and the sinking food usually needs to go in after the lights are off.

Gut length tells you the diet type.
Herbivores have long, coiled intestines built to extract nutrition slowly from bulky plant material, and they are adapted to graze continuously rather than to take large meals. Carnivores have short, simple guts and cope well with fewer, larger, protein-dense meals. Feeding either pattern to the other causes problems, and the herbivore given a carnivore diet is the one that gets seriously ill.

Species that need something specific.
Plecos and other wood-rasping catfish need cellulose and vegetable matter, and many need real wood in the tank, not simply an algae wafer. Herbivorous rift lake cichlids need a low-protein, algae-based food. Predatory species need whole-animal protein rather than plant nitrogen. Marine grazers need macroalgae or a food built around it.

Buoyancy.
Floating pellets are convenient because you can see what is eaten. In deep-bodied fancy goldfish, gulping air at the surface with a floating pellet that then swells in the gut is associated with buoyancy trouble, and pre-soaking the pellet or using a sinking one is a standard first step. For shy or slow species, a slow-sinking food gives them time.

Particle size against gape.
A fish cannot eat what does not fit in its mouth, and it will not persist with something it cannot break up. Small tetras, rasboras and fry all need genuinely small particles rather than crushed adult flake, which fractures into pieces that are still too big and into dust that pollutes.

What changes with life stage, and what changes more

Fry.
Highest demand for protein and energy relative to body mass, and the least capacity to take it in at once. They need very small particles and many feeds a day. First foods are chosen by mouth size rather than by nutritional analysis, which is why liquid fry foods, infusoria and freshly hatched brine shrimp exist as a graded sequence. Crushed adult flake is the most common reason fry stall.

Growth.
Still high protein, still frequent, but the food must now match the adult's feeding level and mouth position as those develop.

Adult maintenance.
Demand falls, and this is where most owners keep feeding at growth rates. The result is fat deposition around the organs and a heavier waste load in the water.

Conditioning for breeding.
Requirement rises again, particularly for the female producing eggs, and this is where richer frozen and live foods have a genuine place.

Older fish.
Intake falls, competition at feeding time becomes harder to win, and softer or smaller particles help. Judge condition by looking down at the fish from above, where loss over the back and behind the head shows long before it does from the side.

Temperature, which matters more than age.
Fish are ectotherms, so the rate at which they digest and use food tracks water temperature rather than the calendar. This is most obvious in ponds, where digestion slows markedly as the water cools and where low-temperature foods built around more digestible ingredients exist for exactly that reason. Feeding a summer ration into cold water leaves undigested food in the fish and uneaten food in the pond. The same principle applies indoors to any species kept cool, and to any tank during a heater failure.

Storage, which changes the food after you buy it

Fat oxidises on contact with air, and fat-soluble vitamins degrade with light and heat. A tub that has been open for many months is not the food described on the label, whatever the best-before date says.

The worst possible storage location is the one almost everyone uses: on top of a lit aquarium hood, which is warm and humid. Keep food in a cool, dark, dry cupboard.

Do not shake food out of the tub while holding it over an open tank. Humid air enters the container each time and the contents cake and spoil faster. Tip a portion into a dry lid or a spoon away from the water.

Buy the size you will use within a few months rather than the size that is cheapest per gram. The saving on a large tub is more than lost to the portion that oxidises before it is fed.

Trust your nose. Fish food that has gone off smells sharp, painty or strongly rancid rather than mild and fishy. Discard it.

Where the usual advice fails

"Feed what they can eat in two minutes."
A reasonable starting heuristic for mid-water fish and useless for anything else. It leaves bottom dwellers and nocturnal species unfed, because the food never reaches them and they are not awake when it does.

"One good food for the whole tank."
Only works in a tank of fish that feed at the same level and on the same diet type. A mixed community usually needs a floating or slow-sinking food and a sinking one, and often a vegetable-based item as well.

"More protein is better quality."
The reason for the warning above. It also raises the nitrogen load in the water, so a high-protein food fed to fish that cannot use it damages the tank as well as the fish.

"Colour food makes them healthier."
It supplies pigment. Whether that helps depends entirely on whether the species deposits carotenoids.

"Crush the adult food for the babies."
Produces particles that are still too large mixed with dust that fouls the water.

"Buy the big tub."
Cheaper per gram, worse per feed.

What never to do

Do not feed a herbivorous cichlid or other long-gut grazer a protein-rich carnivore food, however well they take it.

Do not keep using food that smells rancid or that has been open for a very long time. Oxidised fat damages the liver and the vitamin content is already gone.

Do not store food on the aquarium hood, on a windowsill, or anywhere warm and humid.

Do not judge a food by the front of the pack. Premium, complete, natural and scientifically formulated have no defined meaning on aquatic feed.

Do not assume all fish in a community tank are eating because the food disappears. Something is eating it. That is a different claim.

Do not switch food abruptly and then attribute any change to the new product without testing the water first. Most sudden feeding changes in an aquarium are water quality problems.

Do not feed bread, biscuit or other human staples to pond fish. They swell, they ferment, and they foul the water.

When to get help

A fish in condition holds a smooth outline from above, with no ridge along the back and no hollow behind the head.

Get species-specific advice the same day for a swollen abdomen with white stringy faeces, particularly in a herbivorous cichlid, because that combination often follows a protein-rich food and progresses to refusal to eat.

Act within days if a fish is eating normally and still losing condition, if the back becomes ridged or the area behind the head becomes hollow, if a fish is repeatedly taking food and spitting it out, or if one species in a community is thinner than the rest.

Before you change anything, test the water. Ammonia, nitrite and nitrate results change the interpretation of every feeding problem, and an appetite change is far more often a water quality signal than a food quality one.

Bring to any consultation: the exact product name, how long the tub has been open and where it is stored, how often and how much you feed, which fish are seen to eat, your most recent water test results, and a photograph of the fish taken from directly above.