Start with 20 gallons minimum, then add 10 gallons per fish

Two goldfish need at least 20 gallons of water to stay healthy. This is not a suggestion — goldfish produce waste rapidly and need room to move without poisoning themselves. A 20-gallon tank is the bare minimum for the first fish; add 10 gallons for each additional goldfish. So two goldfish require 20 gallons, three require 30 gallons, and so on.

Many people keep goldfish in bowls or small tanks and watch them survive for months. Survival is not the same as health. Goldfish in undersized tanks develop stunted growth, cloudy eyes, and fin damage from ammonia buildup. They may live for years in poor conditions, but they will not thrive, and their lifespan shrinks from the typical 10 to 20 years down to just a few.

The 20-gallon rule applies to common goldfish and comet goldfish — the orange fish sold in most pet stores. Fancy goldfish (the round, short-bodied varieties with flowing fins) need even more space: 30 gallons for the first fish and 20 gallons for each additional one. If you are unsure which type you have, ask the store or check the fish's body shape — fancy goldfish look rounder and more compact.

Key Takeaways

  • Two common or comet goldfish need a minimum of 20 gallons; fancy goldfish need 30 gallons for two.
  • Tank size matters because goldfish produce large amounts of waste, and undersized tanks develop toxic ammonia levels within days.
  • A 20-gallon long tank (30 inches wide) is better than a 20-gallon tall tank (24 inches wide) because goldfish need horizontal swimming space, not vertical depth.
  • Filtration must handle at least three times the tank volume per hour — a 20-gallon tank needs a filter rated for 60 gallons per hour or higher.

Why tank length matters more than height

Goldfish are bottom feeders that spend most of their time cruising the length of the tank looking for food. They do not use vertical space the way other fish do. A 20-gallon tall tank (usually 24 inches wide and 30 inches tall) feels cramped to a goldfish because the fish cannot swim in a straight line without turning around constantly.

A 20-gallon long tank (usually 30 inches wide and 18 inches tall) gives the same volume but in a shape goldfish actually use. If you are buying a new tank, choose a long model over a tall one. If you already own a tall tank, it will work, but your goldfish will be more active and less stressed in a longer one.

Filtration is as important as tank size

A 20-gallon tank with weak filtration will develop ammonia and nitrite spikes within days. Goldfish produce roughly three times as much waste as other fish their size, so the filter must be oversized. A good rule is to choose a filter rated for at least three times your tank volume per hour. For a 20-gallon tank, that means a filter rated for 60 gallons per hour (GPH) or higher.

Hang-on-back filters (the boxes that clip to the rim) are common and affordable, but canister filters (the cylinders that sit under the tank) handle goldfish waste better because they hold more media and move water more quietly. If you use a hang-on-back filter, clean it every two weeks. If you use a canister filter, clean it every three to four weeks. Either way, never rinse the filter media in tap water — use water from the tank itself, or you will kill the bacteria that break down ammonia.

Water changes and tank maintenance

Even with a strong filter, goldfish tanks need a 25 percent water change every week. This removes dissolved waste that the filter cannot catch. In a 20-gallon tank, that means removing and replacing 5 gallons of water each week. Use a gravel vacuum to suck debris from the bottom while you drain — goldfish stir up the substrate constantly, and waste accumulates fast.

Test the water every week for the first month, then every two weeks after that. You are looking for ammonia and nitrite, both of which should read zero. Nitrate (the end product of the nitrogen cycle) can be higher — up to 40 parts per million is acceptable, but above that, increase your water change frequency. If ammonia or nitrite ever shows up on the test, do a 50 percent water change when ready and check your filter.

Tank mates and decorations

Goldfish eat smaller fish and uprooted plants. Do not add tetras, guppies, or other community fish to a goldfish tank — they will disappear. Goldfish can live with other goldfish of similar size, but they will nip fins and compete for food, so watch them closely during the first week. If aggression develops, separate them.

Decorations should be smooth or soft. Goldfish have delicate eyes and fins, and sharp rocks or plastic plants will tear them. Smooth river rocks, driftwood, and live plants like anubias (which goldfish usually ignore) work well. Avoid gravel smaller than pea-sized, because goldfish will suck it up while feeding and may choke. Sand works fine if you prefer it, but it traps waste and needs more frequent vacuuming.

Temperature and lighting

Goldfish are coldwater fish and do not need a heater. They thrive between 65 and 72 degrees Fahrenheit. If your home stays warmer than 75 degrees year-round, you may need a chiller, but most homes stay in the right range naturally. Avoid placing the tank in direct sunlight, which causes algae blooms and temperature swings.

A basic LED light on a timer (8 to 10 hours per day) is enough. Goldfish do not need special lighting, and bright light for too long encourages algae. If algae becomes a problem, reduce the light duration to 6 hours per day and increase water changes to twice weekly.

Common mistakes that stunt goldfish growth

The most common mistake is overfeeding. Goldfish have no stomach and will eat until they are bloated. Feed them once or twice daily, only as much as they can eat in two minutes. Uneaten food rots and spikes ammonia. Frozen peas (thawed and squeezed out of their shells) are a good weekly treat that helps with digestion, but pellets should be the main diet.

The second mistake is changing too much water at once. Goldfish are hardy, but sudden temperature or pH swings stress them. Keep new water within 2 degrees of tank temperature and add it slowly — over 10 to 15 minutes if possible. The third mistake is neglecting the filter. A clogged filter looks like it is working (water still flows), but it cannot process waste. Clean it on schedule, not just when you remember.

Frequently Asked Questions

Can I keep two goldfish in a 10-gallon tank?

No. A 10-gallon tank is too small for even one common goldfish. Ammonia builds up within days, and the fish will develop health problems within weeks. The minimum for two goldfish is 20 gallons. If space is limited, consider a different species — bettas, tetras, or corydoras catfish thrive in smaller tanks.

Do I need a heater for goldfish?

No. Goldfish are coldwater fish and prefer temperatures between 65 and 72 degrees. A heater is unnecessary and can stress them. If your home is consistently above 75 degrees, a chiller may help, but most homes stay in the right range without one.

How often should I clean the tank?

Do a 25 percent water change every week and vacuum the gravel at the same time. Clean the filter every two to four weeks depending on the type. Never do a full tank cleaning — that kills the bacteria that keep the water safe. Only clean the filter media in old tank water, never in tap water.

What do I feed two goldfish?

High-quality pellets made for goldfish are the main diet. Feed once or twice daily, only as much as they eat in two minutes. Add a thawed frozen pea once a week as a treat. Avoid flake food (it clouds the water) and live food from the wild (it may carry parasites).

Will my goldfish grow to fill the tank?

No. Goldfish grow to their genetic size, not the tank size. A common goldfish grows to 10 to 12 inches regardless of tank volume. However, in a tank that is too small, they grow slower and develop deformities. A larger tank lets them reach their full size and live longer.