Fish tanks range from less than one gallon to 200 gallon plus. So, the size of your aquarium tanks not only depends on the type and number of fish you intend to keep, but also the space you have available. As a general rule of thumb, a five-gallon tank is the minimum recommended for a single fish as smaller tanks and bowls can be susceptible to fluctuating PH levels and a build-up of waste materials which can be harmful for its inhabitants. The most popular home aquariums are between 10 and 20 gallons, as they comfortably hold at least a couple of fish, plus plants, accessories and the necessary fish care devices. A 20 to 40-gallon tank can comfortably hold half a dozen fish, while for multi-fish and multi-species aquarium set ups, a tank size of over 40 gallons is advised.