What Size is Best for a Goldfish Tank?

In this guide

Goldfish have one of the worst reputations in the aquarium hobby for being kept in spaces far too small for them, and the fishbowl image most people grow up with is close to the opposite of what a goldfish actually needs. They are large, long lived, messy fish, and getting the tank size right from the outset saves a great deal of grief later.

Why goldfish need so much more space than people expect

Goldfish are carp, closely related to koi, and share the same tendency to keep growing well past the size most shops sell them at. A goldfish that looks perfectly happy in a two foot tank as a juvenile can reach 20 to 30 centimetres as an adult, and produces a surprising volume of waste for its size along the way. Undersized tanks lead to poor water quality between water changes, stunted and stressed fish, and a shortened lifespan for an animal that can otherwise live well over a decade.

Minimum sizes worth working with

• Single-tailed goldfish (common, comet, shubunkin): at least 115 to 150 litres for the first fish, with 40 to 55 litres more for each additional fish, since these varieties are fast growing and can reach 30 centimetres or more
• Fancy goldfish (oranda, ryukin, fantail, telescope): at least 75 to 115 litres for the first fish, with 35 to 40 litres more per additional fish, since fancy varieties stay smaller but produce disproportionate waste for their size
• Never a bowl or any tank under about 40 litres, regardless of the goldfish variety or its current size

Shape matters as much as volume

A wide, shallow tank generally suits goldfish better than a tall, narrow one, since most goldfish spend their time in the lower and middle part of the water column and benefit from more horizontal swimming room and greater surface area for gas exchange, rather than height they rarely use.

Filtration has to match the bioload

Whatever the tank size, filtration needs to be rated well above what the tank volume alone would suggest, since goldfish are unusually heavy waste producers. Aim for a filter that turns over the entire tank volume eight to ten times every hour, well above the four to six times an hour that suffices for a typical lightly stocked tropical community tank, alongside a weekly water change routine, since goldfish tanks tend to accumulate ammonia and nitrate faster than a similarly stocked tropical community tank.

Buying for the adult, not the juvenile

The single most common mistake with goldfish is buying a small tank to match a small, newly bought fish. Plan and buy for the size the goldfish will reach as an adult, not the size it is in the shop, since upgrading a tank that already has an established, settled fish in it is far more stressful for everyone involved, fish and keeper alike, than starting with adequate space from day one.

A note for Indian buyers

Many Indian aquarium shops still sell goldfish alongside small bowls and round tanks explicitly marketed for them, which sends exactly the wrong message about what these fish actually need. Tank sizes in India are usually listed in litres rather than gallons, so it is worth converting any gallon-based guidance you come across, including the figures above, before buying, rather than assuming a locally sold "medium" tank is automatically large enough.

Article content © Ravi Teja. All rights reserved.