A goldfish grows to match the water it lives in. This guide covers how to choose a tank size for your variety, how to filter the waste goldfish produce, and how to feed without fouling the water.
Run these five points for any goldfish tank. Each one is covered in the sections below.
Essential for long-term health
Determines waste dilution
Handles heavy waste
Prevents ammonia spikes
Prevents water fouling
Common and comet goldfish have long, streamlined bodies. They swim fast and grow large, so they need the longest tank and the most water.
Fantail and lionhead goldfish have short, rounded bodies and swim more slowly. Their size still matters, because they produce waste that the tank's water has to handle.
Plan around the adult size of your variety, not the size of the fish in the store.
Waste builds up quickly in a small volume of water. Ammonia and nitrite rise between water changes, and poor water quality stresses the fish.
Stressed goldfish eat less and grow more slowly. The practical rule is to give the water more volume than the fish needs today.
Use this method for any variety. It takes about five minutes with a tape measure.
Write down the variety and the adult size you are planning for.
Measure the tank's inside length, width, and height in inches.
Multiply length by width by height, then divide by 231. The result is the gross volume in US gallons.
Check that the tank's length gives the fish room to turn and swim. A tall, narrow tank holds the same gallons as a longer one but gives less swimming room.
A tank 36" × 18" × 16" = 44.9 US Gallons
Note: This is gross volume. Substrate, decorations, and the water lost at the top reduce the usable volume.
Choose a filter rated for a larger tank than yours. Goldfish produce a lot of waste for their size.
Use mechanical media to trap solid waste efficiently before it breaks down.
Use biological media to house the bacteria that break down ammonia into less toxic nitrate.
Rinse biological media in tank water, not tap water, so the beneficial bacteria survive.
Plan a regular partial water change on a fixed schedule to maintain systemic stability.
This guide does not cover specific filter models.
Check turnoverSinking pellets suit goldfish because they reach the bottom, where goldfish also feed. Floating food is taken at the surface, where goldfish also swallow air.
Uneaten food decays and adds ammonia to the water, so a smaller portion is the simpler fix.
A new tank has no bacteria to break down ammonia. A fishless cycle builds that bacteria population first.
Add a measured ammonia source, test the water, and wait until the tank converts ammonia to nitrite and nitrite to nitrate before adding fish.
A bowl holds little water and usually has no filtration, so waste and ammonia build up quickly. A filtered tank of adequate volume is much easier to keep stable.
Growth slows when water quality declines, and a small tank declines faster. Match the tank volume to the adult size and keep the water tested.
Watch for ammonia or nitrite readings that stay above zero between water changes, cloudy water, fish gasping at the surface, and slow growth. Any of these means the tank is working harder than it should.
A liquid test kit that reads ammonia, nitrite, and nitrate gives you the numbers to check the cycle and your water changes.
Use the tank volume calculator to get the gallon figure. Then check filtration and the cycle against that number.