How much water do goldfish need?
A good rule of thumb for goldfish is three gallons of water per inch of body length for fat, fancy goldfish and two gallons per inch for comets. This means most fish will eventually require 15-20 gallons (60-75 liters) of water each. However, a fifteen or twenty gallon tank will not provide adequate swimming space for an adult goldfish. For fancies, a 29 or 30 gallon (108 liter) tank is a minimum. For the more active comets, a four foot (120 cm) or larger tank is better. In ponds where there is more debris blown in, plan on at twice as much water per fish.
