🐡 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. 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 72°F (18 to 22°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 lists them among the species it is illegal to keep singly. 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.

🛒 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 questions