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Goldfish care: the tank they actually need, and why the bowl kills

A goldfish is a hand-sized carp that lives twenty years and produces more waste than any tropical fish. The tank size, filtration, feeding and water routine that let it get there.

What a goldfish actually is

A goldfish is a domesticated carp. Left to grow, a common goldfish reaches 25 to 30 cm and a fancy goldfish 15 to 20 cm, they live fifteen to twenty years, and they eat and excrete like the pond fish they are. The 5 cm fish in the shop is a baby, and the bowl it is sold next to is the reason most goldfish die before their first birthday and the reason “goldfish only live a year” is something people believe.

They are also, kept properly, one of the best fish in the hobby: intelligent, long-lived, tame enough to eat from a hand, and available in more shapes and colours than any other species. The list below is what “properly” means, and the tank size is the part that surprises everyone.

The fish is the same. The volume decides whether its own waste poisons it between water changes.

The tank

A hundred litres for the first fancy goldfish, and 40 to 50 more for each one after. Commons, comets and shubunkins, the single-tailed, fast-growing kinds, want 200 litres or a pond. This is not about swimming room, though they use it; it is about waste. A goldfish produces several times the ammonia of a tetra its size, and the volume is what dilutes it between water changes.

A bowl is not an aquarium. No filter, no oxygen exchange to speak of, no room, and a temperature that follows the room. A goldfish in a bowl is stunted by its own hormones, poisoned by its own waste, and dead in months. There is no bowl big enough, and no goldfish small enough.

Filtration rated for twice the tank. Two sponge filters, a canister, or a large hang-on-back: goldfish need more bacteria than any tropical tank of the same size, and they need the water moving and the surface broken for oxygen. Cycle the tank before the fish arrive, the fishless way; a goldfish added to a new tank is the worst possible case of new tank syndrome, because it produces the ammonia of five tropical fish.

No heater. Goldfish are cold-water fish, happiest at 18 to 22 °C and fine lower. A heated tropical tank speeds their metabolism, shortens their life, and rules out every tropical tankmate anyway.

A bare bottom or sand, not gravel. Goldfish forage by mouthing the substrate, and pea gravel is exactly the size that lodges in a goldfish’s throat. Sand is safe and shows waste so it gets siphoned. Plants get eaten or uprooted; anubias and java fern tied to wood survive, most others do not, and floating plants are a salad.

Water

The parameter bands are the usual ones, but a goldfish tank reaches them faster: ammonia 0, nitrite 0, nitrate under 20 ppm takes a 30 to 50 % water change every week, not the 25 % a tropical tank gets away with. Test weekly, before the change, and watch how fast nitrate climbs; if it passes 40 ppm between changes, the tank has more goldfish than it can carry, and the stocking guide explains why a bigger filter will not fix that. Vacuum the sand with every change: goldfish waste is where next week’s ammonia comes from.

In the app — a goldfish tank’s readings move fast, and the nitrate trend is where you see it: the schedule advisor proposes a tighter water-change cadence when nitrate climbs 20 ppm across recent tests, and the calculator sizes the change to the tank’s volume and shows where it lands the number.

Log this week’s test →

Feeding

Goldfish are omnivores and they are always hungry, which is not the same as needing food. Small amounts twice a day, gone in a minute, and one fast day a week. Overfeeding is the goldfish problem: it clouds the water, spikes ammonia, and, in fancy goldfish, blocks the gut and sets off the floating and sinking of swim bladder trouble.

Sinking pellets, not floating flakes. Fancy goldfish gulp air at the surface with floating food, and air is the other cause of floating. Pre-soak pellets for a minute so they swell in the bowl rather than in the fish.

Vegetables. Blanched peas with the skin off, spinach, courgette, and lettuce clipped to the glass. The fibre keeps a goldfish’s gut moving and a goldfish that gets greens weekly rarely has buoyancy problems. A cooked pea is also the first treatment when one does.

Fancy or single-tail: not together

Fancy goldfish (orandas, ryukins, fantails, ranchus, black moors, telescopes) have compressed bodies bred for shape. The organs inside are squeezed, which is why they float, sink and get constipated more than any other fish, and why the feeding rules above matter twice for them. They are slow, and some have poor eyesight.

Single-tailed goldfish (commons, comets, shubunkins) are the fast, streamlined, pond-sized kind. Kept with fancies they take all the food and the fancies starve; kept in a small tank they stunt. They belong in a pond or a very large tank, and not with the round ones.

The problems, in the order they arrive

Cloudy water and ammonia in a new tank. The goldfish equivalent of the first month. Test daily, change water whenever ammonia or nitrite reads above 0.25 ppm, and read the ammonia guide if the fish is at the surface.

Floating, sinking, or upside down. Nearly always food or air in a fancy goldfish: fast for two days, a pea, then sinking pellets and greens. The swim bladder guide is the full plan, and the infection case is the one that does not improve after a week.

Red streaks in the fins, bloody patches, clamped fins. Ammonia burn. Test, change half the water, and find the cause; it is usually feeding or a filter that cannot keep up.

White spots after a cold snap or a new fish. Ich, and the ich guide applies, with one difference: do not raise a goldfish tank above 24 °C, so the treatment leans on the medication and gravel vacuuming rather than heat.

Fin rot and fungus. Water quality again, in a tank that has been skipping changes. The fin rot guide covers it; the fix is always the water first.

Tankmates

Other goldfish, of the same body type. That is the honest list. Tropical fish need a heater goldfish should not have; plecos and other “algae eaters” suck the slime coat off a slow goldfish at night; small fish are food once the goldfish is large enough; and anything slow and long-finned gets outcompeted. White cloud mountain minnows and nerite snails are the two cold-water exceptions that work in a large tank. A goldfish is not lonely on its own, but a pair or a group of the same kind is more active and more fun to watch.

In the app — add each goldfish as an inhabitant and log its length every month or two: the growth chart is the best early warning for a tank that is too small, because a healthy young goldfish that has stopped growing is telling you about its water. Stocking ideas will keep a goldfish tank to goldfish.

Add your goldfish and track its growth →

Common questions

How big a tank does a goldfish need?

A hundred litres for the first fancy goldfish and 40 to 50 litres for each additional one; 200 litres or a pond for single-tailed commons and comets. The number is set by waste, not swimming: a goldfish produces several times the ammonia of a tropical fish its size, and the volume is what keeps the water safe between weekly changes. A bowl is not on the scale at all.

How long do goldfish live?

Ten to fifteen years is normal in a proper tank, and twenty is not rare; the record is over forty. The “goldfish only live a year” belief comes from bowls, where they die of ammonia and stunting long before old age. A goldfish that dies in its first year almost always died of its housing.

Do goldfish need a filter and a heater?

A filter, absolutely, and a big one: goldfish are the heaviest waste producers in the hobby and need more bacteria and more oxygen than a tropical tank of the same size. A heater, no. They are cold-water fish, happiest at 18 to 22 °C, and a heated tank shortens their life and rules out the tank for them and for tropical fish both.

Why does my goldfish float or sink?

Usually food: a blocked gut or gulped air pressing on the swim bladder, which fancy goldfish with their compressed bodies are built to suffer. Fast for two days, feed a cooked shelled pea, switch to sinking pellets and add greens, and most cases fix themselves within a week. A fish that is still off balance after that, and lethargic with it, needs a checkup and probably an antibacterial.