The links below detail the main standard tank sizes which are available in the UK and the US, and which may also correspond to those available in other parts of the world. Remember that nominal capacities quoted for tanks are greater than the actual volume of water, due to the normal fill level being an inch or two from the top, depth of substrate and bogwood, rocks, etc used as decor. The volume may therefore be more accurately determined when filling the tank initially.

The stocking levels quoted have been based on two different systems which are used. The first is based on the volume of water required to support each fish. The second system is based on surface area of the tank, which, as it represents the area available for oxygen to dissolve into the water, is considered by some to be a useful indicator of maximum stocking levels. With this system, a deeper tank with the same base dimensions cannot hold more fish.

Remember, these are only guidelines, which work reasonably well for general community tanks as a rough guide, and a certain amount of common sense must be applied - you can't keep two 10" fish in a 20 gallon tank for instance! It is also sensible to understock the tank to allow a safety margin - it will be much easier to keep the fish healthy in a lightly stocked tank, as well as reducing the amount of filter maintenance and water changes that need to be done.