Goldfish need far more space than the bowls they are commonly associated with. A single fancy goldfish requires a minimum of 20 gallons, with 10 additional gallons for each extra fish. Common and comet goldfish grow over 12 inches and belong in 55-gallon or larger tanks, or ideally outdoor ponds.

Goldfish are coldwater fish that thrive between 65 and 75 degrees Fahrenheit. They do not need a heater in most indoor environments, but they do need a thermometer to monitor for dangerous temperature swings, especially in rooms with direct sunlight or poor climate control.

Filtration is critical because goldfish produce substantially more waste than tropical fish of similar size. Choose a filter rated for at least double your tank volume. Canister filters and hang-on-back filters with large media capacity work well. Sponge filters provide supplemental biological filtration and are gentle enough for fancy varieties with delicate fins.

Goldfish are active swimmers that need horizontal swimming space more than tank height. Long rectangular tanks are better than tall narrow ones. Avoid sharp decorations that can tear flowing fins on fancy varieties.

Substrate should be either fine sand or large river rocks. Medium-sized gravel is a choking hazard because goldfish sift through substrate while foraging and can swallow pieces that cause intestinal blockages.

Goldfish uproot most live plants, but hardy species like anubias attached to rocks and java fern tied to driftwood can survive their attention. Silk plants offer a safe decorative alternative.

Weekly water changes of 25 to 30 percent are essential given their heavy waste output. Test water regularly, keeping ammonia and nitrite at zero and nitrate below 40 parts per million for optimal health.