Guidelines are recommendations by experienced goldfish keepers for those of lesser (or no) experience. They are not the only way of keeping goldfish, or even the best, but are our compromise between ideal conditions for goldfish and the space, time, and money limitations of the goldfish hobbyist. Koko’s guidelines are continually reviewed and updated as needed by the moderating team as we gain new information from research and experience.
Optimally, your tank should be large enough to provide at least 20 gallons for each goldfish, with a minimum of 15 gallons per fish. This is true for both long-bodied and fancy goldfish. While baby goldfish can do well in a smaller tank short term, they grow very fast, so we recommend starting them in their grown-up tank.
The ideal tank for goldfish is shallow with a large surface area. Tall tanks should be avoided if possible.
The most common filters for goldfish tanks are HOBs (hang on the back) or canisters. We recommend a HOB filter be rated by the manufacturer as turning over at least 10 times the tank volume per hour. Thus a HOB filter for a 20 gallon tank should turn over at least 200 gph. If one has multiple HOBs the turnover rates should add up to at least 10x the tank volume per hour.
Canisters have more filter volume, and we recommend they turn over at least 5-7 times the tank volume per hour.
There are many other types of filters, including many DIY filters. If you are using any of these, please start a thread in the Water Quality forum to ask if your particular filter is appropriate for your set up and your fish load.
We recommend a minimum of a 50% water change each week. This assumes that you are meeting the recommendations for tank size and filtration.
Aeration is not usually needed if your tank and filter(s) are large enough for your fish load, the water surface area is large enough to provide good gas exchange, and the fish are healthy. Even then, adding aeration is generally beneficial. We will often recommend adding aeration when treating sick fish.
Goldfish are called “cold water” fish, not because they require cold water, but because they do not require heated tanks like tropicals. Generally, healthy goldfish prefer the same temperatures as we do and are happy at room temperature (including a cooler night temperature). However, we may recommend heating the tank when treating sick fish, so it is useful to have a heater available.
Goldfish tanks may have no substrate, sand, or gravel. We recommend that hospital and quarantine tanks have no substrate for ease and completeness of cleaning. If one uses sand or gravel in the main tank, the layer should be no more than ½ inch thick. Gravel should be either small enough that the fish can easily swallow it or too large to fit in the fish's mouth.
Avoid hollow ornaments. The water in these can become anaerobic and produce toxins. Avoid ornaments that have sharp points or spaces that can trap a goldfish.
We do not recommend routine use of salt in a goldfish tank. Salt is a valuable, gentle medication which will lose some of its value if it is in the water on a regular basis.
We recommend quarantining all new fish for at least a month before introducing them to the “old” fish. During this quarantine period, we treat with salt and praziquantel.
Goldfish do best with other goldfish as their tank mates. Most other fish are likely to harm and/or be harmed by goldfish. Please ask about their suitability before adding any other kind of fish to your tank. Even fish that get on well in a goldfish tank, such as bristle nose or rubber lip plecos, or even large snails, produce waste. So adding these reduces the number of goldfish your tank can support.
Here are some articles with additional information about keeping goldfish.