Goldfish, Carassius auratus auratus, are one of the if not the most popular aquarium fish ever. Most fishkeepers get involved with fishkeeping with goldfish. Unfortunately most of the time they start with a ten-gallon aquarium at best. Many features make these fish a good choice, but there are certain needs that need to be fulfilled. This article will focus on fancy goldfish, the round-bodied varieties.


One of the major issues, and most often over-looked, is tank size. These fish get much larger than the little one-inch babies swimming around the dealer’s tanks. There is some variation in size based on breed. In general they can reach fist size for their body alone, not including their fins. That is a large fish compared to what most people bring home. They are also schooling fish. They like the company of other goldfish, so a school should be established. A school is usually a minimum of six fish. One of the best guides for determining minimum tank size for goldfish is to start with 20 gallons for one and add an additional 10 gallons for each additional fish. This puts the minimum tank size for a school of goldfish in the 55-75 gallon range, and bigger is always better.



Another important aspect is water quality. This is determined by filtration and water change schedule. Filtration keeps the water clean between water changes and collects the debris for the fishkeeper to remove. Goldfish are not necessarily dirtier than other fish for their size. They are about equal to cichlids weight for weight. But lots of filtration is needed for goldfish. The best form of mechanical filtration is canister filters because they force the water through the media. In Hang-on-back (HOB) and other types of filtration only gravity forces the water through the media. So when the media is clogged the water goes under, over, or around the media rather than through it. In a canister, clogged media will reduce the flow rate, rather than simply letting water that has not been cleaned back into the tank. It is important to clean any filter at least monthly. Many people wait until the flow is reduced, and some filters claim they can go months without cleaning, but the debris in the filter will breakdown into ammonia and eventually nitrate, reducing water quality and greatly increasing nitrate concentration. In general, whatever a filter claims it can handle should be cut in half. This should be considered the absolute minimum for goldfish. The more filtration there is on the aquarium, the better. There is no such thing as too much filtration. There can be too much flow. Even in heavily filtered aquariums the aquascaping usually reduces flow, creating areas of higher and lower flow. Most goldfish can deal with moderate flow rates, especially when offered lower flow resting areas. There are some breeds and individuals less tolerant of higher flow, so when there is evidence of stress due to too much water flow, the appropriate changes to the aquarium’s setup and filtration should be made.

Water Changes:

Water changes are the most important aspect of water quality. Weekly water changes are the best way to keep water quality up. The minimum water change schedule is determined by the nitrate concentration, which should be maintained at no more than 40ppm with a maximum of 20ppm being a much better standard. This usually ends up being somewhere in the range of 25-50% per water change. Nitrate can reduce growth and stress the fish. There are other chemicals in the water that can have the same affect. In general if nitrate concentration is within a safe range, the other possibly problematic chemicals will be too.


Goldfish are very hardy and can tolerate a wide temperature range. In general room temperature is ideal. The most common exception is when the room’s temperature varies so much that it can be stressful to the fish. In this case it is best to get a heater and set it at the higher end of the room’s range. The higher the temperature is, the higher their rate of metabolism. This means more waste and a higher need for oxygen. Also, the warmer the water the lower the amount of oxygen it can carry. When temperatures do get high watch for the fish gasping at the surface, this would show that they are not getting enough oxygen.


In general every tank should have an air pump and air stone. If there is enough surface agitation an air pump and air stone may not truly be NEEDED, but in general fish seem to do better with an air pump. They seem to be more active and have fewer health problems when they have an air pump. Some goldfish even enjoy playing in the bubbles.


There are many options for substrates in the aquarium, but not all are suitable for goldfish. It may seem as little more than a decoration to make the tank look better to us, but the substrate can have a huge impact on water quality, fish health, and their well being.

Pea-sized Gravel:

The most common substrate in freshwater aquariums is pea-sized gravel. This is not a good option for goldfish. For one, as in any tank, any sized gravel can trap a lot of debris which can cause water quality and health problems. As debris breaks down it becomes ammonia and eventually nitrate. It may even serve as a breeding ground for parasites, harmful bacteria, and other harmful organisms. Lots of debris in the substrate can lead to infestations of Planaria worms, a type of flatworm. One of the biggest threats of pea-sized gravel is that once the goldfish are big enough, they can actually swallow the gravel. The gravel can get caught in the mouth, throat, or digestive system. There is little a fishkeeper can do in this situation unless the piece of gravel is in the mouth, but even then there is little that can be done and removing the piece of gravel can cause damage to the mouth and possibly lead to infection or permanent injury.