I have 4 golsfish in a standard US 20 gallon long tank. The filter I have is a Whisper 30-40 gallon filter, AND an undergravel filter powered by 2 powerheads. The water quality stays good, with 1/2 water changes once a month. The three fancy goldfish in the tank are each about 2.5 inches long, not including tail fns, and the one shubunkin in there is about 5 inches long, not including tail. They seem happy and are disease free, but the tank is not really big enough, which is why I have such intense filtration. Many sites recommend 10-20 gallons per fish.