Thriving best in a large aquarium, they carry a heavy bio-load and are easily the messiest fish species. You can’t slack on water changes with these guys, especially in a heavily stocked aquarium. A 25% water change every 2 weeks is a must.
Always add aquarium salt to the water. It helps balance the osmotic qualities of the round fish body. It also helps lower stress and make it easier for internal organs to function normally. This is one of the most important tips for proper goldfish care.
As a cold water species, they do not require a heater. A prime temperature is around 70 degrees. (As a comparison, tropical fish require temperatures around 76 degrees). Goldfish to not do well in warm temperatures which is the main reason why goldfish & tropical fish should never be combined.
Fancy goldfish need lots of space, consider an aquarium larger then 30 gallons. Even though they are small when young, the maximum size is quite large. Keeping them in smaller fish tank can stunt growth which usually results in premature death.