Additional information (Wiki): Like most carp, goldfish produce a large amount of waste both in their faeces and through their gills, releasing harmful chemicals into the water. Build-up of this waste to toxic levels can occur in a relatively short period of time, and can easily cause a goldfish's death. For common and comet varieties, each goldfish should have about 20 US gallons (76 l; 17 imp gal) of water. Fancy goldfish (which are smaller) should have about 10 US gallons (38 l; 8.3 imp gal) per goldfish.

The water surface area determines how much oxygen diffuses and dissolves into the water. A general rule is have 1 square foot (0.093 m2). Active aeration by way of a water pump, filter or fountain reduces the minimum surface area.