🐡 Goldfish Tank Setup: Size, Filter, and Why No Heater

A goldfish tank goes wrong in three predictable ways: the tank is too small, the filter is too weak, and somebody adds a heater. Get those three right and a goldfish becomes one of the hardiest pets a family can keep — the record holder, a fairground fish named Tish, lived to 43.

Twenty gallons for the first fish

Start at 20 gallons for one goldfish and add roughly 10 gallons for each fish after that. That sounds enormous for a fish that costs a few dollars, and it is the most important number on this page. Goldfish are not small animals: common and comet goldfish reach 10 to 12 inches and are really pond fish that were never meant for a desktop tank. That does not mean a park pond, though: never release a goldfish into the wild, where they grow huge, breed fast, and crowd out the native fish. Fancy varieties such as the fantail, oranda and ryukin stay closer to 6 to 8 inches and are the better choice indoors, though they swim slowly and get out-competed at feeding time. A wide tank beats a tall one, because surface area is what lets oxygen in.

No heater, but keep a thermometer

Goldfish are coldwater fish. They are comfortable at ordinary room temperature, roughly 65 to 75°F (18 to 24°C), and a tropical heater set to 78 speeds up their metabolism, shortens their life and leaves less oxygen in the water. Skipping the heater is not the same as ignoring temperature: keep the tank out of direct sunlight, away from radiators and off a drafty windowsill, because a sudden swing is far more dangerous than a steady cool. A stick-on thermometer costs almost nothing and catches the problem early.

Filter for a messy fish

Goldfish eat constantly and produce a lot of waste, which is why a tank that looks perfectly clear can still be quietly poisoning them with ammonia. Choose a filter rated for a tank at least twice the size of yours — a filter sold for 40 gallons on a 20-gallon goldfish tank is not overkill, it is about right. Aim the outflow along the glass or into a decoration so the current stays gentle, which matters most for fancy goldfish, whose trailing fins make them weak swimmers.

Cycle the tank before the fish comes home

A new tank has to grow the bacteria that process waste before it can keep anything alive, and that takes about four to six weeks. Set the tank up, add a pinch of food or bottled ammonia to feed the bacteria, and test the water until ammonia and nitrite both read zero. Our freshwater aquarium setup guide walks through cycling step by step. Buying the fish on the same day as the tank is the single most common reason a first goldfish dies in week two.

Feeding, and the floating-flake problem

Use sinking pellets rather than floating flakes. A goldfish gulping at the surface swallows air, and trapped air is a common cause of the swim bladder trouble that leaves a fish floating sideways or upside down. Feed only what they finish in about a minute, once or twice a day, and offer a blanched, deshelled pea once a week to keep things moving. Overfeeding is the other half of the ammonia problem, because uneaten food rots where it lands.

The weekly routine

Change about a third of the water every week, using dechlorinated water at roughly the tank temperature. Vacuum the gravel while you do it, since that is where waste collects. When the filter slows, rinse the media in the old tank water you just removed and never under the tap — chlorine kills the bacteria you spent six weeks growing.

Tankmates, and who not to add

The safest company for a goldfish is another goldfish. They are sociable, and Switzerland's animal welfare rules say goldfish should be kept in company. Do not mix them with tropical community fish, which need warmer water, and do not add a betta — wrong temperature, and bettas flare at trailing fins. Small fish and shrimp tend to be treated as snacks.

Before you buy

A goldfish is a 10 to 20 year commitment that begins at about three dollars, and the tank, filter and stand cost many times more than the animal. That is not a reason to skip it. It is the reason to set it up properly the first time. And if the day ever comes when you cannot keep yours, rehome it — a pet shop, a local fish club or a school will usually take it.

🛒 Recommended supplies

Hand-picked gear for this guide. Affiliate links — we may earn a commission. The $/$$/$$$ badges are a rough budget guide, not live prices.

🧽 Cleaning & maintenance

The main job is regular water changes. Each week, use a gravel vacuum (siphon) to remove about 20–30% of the water along with waste from the bottom, then refill with fresh water treated with a dechlorinator so it is safe. Scrape algae off the glass as needed and rinse the filter media in old tank water — never tap water, which kills the good bacteria. Test the water so you can catch problems early. Never do a full strip-down that wipes out the filter bacteria; steady partial maintenance keeps the water stable and your goldfish healthy.

Cleaning supplies for this habitat. Affiliate links — we may earn a commission. The $/$$/$$$ badges are a rough budget guide, not live prices.

⚠️ Common goldfish setup mistakes

Already have one and something feels off? Start here. These catch almost everybody — none of them mean you have done badly, and every one of them is fixable.

Does a common goldfish need a pond?

The mistake: Keeping a common goldfish in a 20-gallon.

The 20-gallon starting point suits the fancy varieties. A common or comet reaches 10 to 12 inches, and the RSPCA's rule of thumb is a tank at least four times the fish's adult body length — roughly four feet long for a grown one.

Try this: A 4-foot tank of about 75 gallons, or an outdoor pond, is what a single-tail goldfish is really asking for. That is a lot of money, so the honest alternative is rehoming to someone with a pond: fish clubs, schools and pond keepers take them, and doing it early beats waiting until the fish is unwell.

Is a 20 gallon tank big enough for two goldfish?

The mistake: Adding a second goldfish without more water.

The 20 gallons covers the first fish only — each one after that needs about 10 gallons more. Two goldfish in a 20 make double the waste in the same water, so ammonia and nitrate climb between water changes.

Try this: Aim for 30 gallons for a pair. If a bigger tank has to wait, add a second smaller water change midweek and test the water so you can see how it is holding up, rather than hoping for the best.

Do I need to quarantine a new goldfish?

The mistake: Adding a new goldfish straight to the tank.

Shop and fairground fish often carry white spot or flukes without showing a thing for a week or two. Once it is in your tank the whole tank has it, and you end up treating fish that were perfectly healthy.

Try this: Keep a new fish on its own for about four weeks first. A clean plastic storage tub with a cheap sponge filter and a thermometer does the job — no gravel, no decorations, easy to watch and easy to empty.

Should I empty the whole tank to clean it?

The mistake: Stripping the tank down for a full clean.

Fresh water, scrubbed gravel and a new filter cartridge take out the bacteria colony along with the mess, and the tank quietly starts cycling again with the fish inside it. A goldfish that looked fine on cleaning day can be ill a few days later.

Try this: Change about a third at a time and leave the green film on the back glass, which is food rather than dirt. Vacuum a different patch of gravel each week, and squeeze the filter sponge out in the bucket of tank water you just removed.

Can a goldfish choke on gravel?

The mistake: Using small pebbles as goldfish gravel.

Goldfish pick stones up, suck the algae off and spit them out again, which is normal and fun to watch. The trouble is a pebble about the size of their mouth, which can wedge behind the lip and stop them feeding.

Try this: Pick gravel too big for your fish to lift, or go bare-bottomed, which is easier to keep clean anyway. If a stone does get stuck and has not come free within a day, ask a fish-savvy vet or an experienced keeper rather than poking at it yourself.

❓ Common questions

📚 Where this comes from

We check our care advice against vets, animal charities, and government health guidance. Here is what this page is built on.

Vets, charities & official guidance

• How to break up with your goldfish — US Fish and Wildlife Service
• Goldfish (Carassius auratus) species profile — USGS
• How should I care for my goldfish? — RSPCA Australia
• Choosing an aquarium for pet fish — RSPCA

Keeper & species-community guides

• Care guide for fancy goldfish — Aquarium Co-Op

Checked August 2026. These are other people’s websites, and none of it replaces your own vet — if a pet seems unwell, call one.