The real answer is: as many as you can squashed in it! But the real question is : how many fish can my tank hold comfortably?
Virtually every fish owner has gazed at their aquarium and wondered just how many fish can be put in there. Unfortunately aquariums don't come with a stocking chart slapped on the side.
As a result many owners unwittingly overstock their tank, sometimes with a disastrous outcome. So how does a fish owner know how many fish they can keep? There are a number of factors to consider, as well as several methods for calculating safe stocking levels.
One Inch Per Gallon Rule
The most widely known rule for stocking a tank is one inch of fish per gallon of water. While this type of calculation works as a rough estimate, it leaves plenty of room for error.
Like people, fish are not all the same size and shape. Stocking a ten-gallon tank with ten inches of slender shaped zebras is not the same as stocking it with ten inches of full-bodied goldfish. Larger bodied fish create far more waste, and therefore require more water volume.
Furthermore, the fish often are not fully grown when first brought home. The adorable little catfish that is scarcely an inch long today could reach a half foot in size when it grows up. The true adult size of the fish must be used in the calculation.
However, many owners have no idea how old their fish is or how large it will grow to be. Before making a decision, always research the fish in question to determine the true adult size.
Another place for error is assuming the size of the tank is equivalent to the number of gallons of water it holds. A ten-gallon tank filled with gravel, rocks, plants, and an assortment of decorations does not hold ten gallons of water. In reality the water volume is often ten to fifteen percent less than the size of the tank.
So while the one-inch per gallon rule is a reasonable yardstick, it has it's flaws.
Surface Area Calculation
The larger the surface area of the water, the greater the oxygen exchange, which in turns supports a larger number of fish. Therefore, surface area of the water directly impacts how many fish can be kept in an aquarium. A tank that is tall and thin may hold the same number of gallons as a tank that is short and wide, yet they have vastly different surface areas.
Using the surface area rule, the shape difference between the tanks is taken into account. The surface area is calculated by multiplying the width times the length of the tank.
Under the water surface area rule the tank can be stocked with one inch of fish for each twelve square inches of surface area. However, this calculation has many of the same flaws as the one-inch rule. For instance, it assumes a fairly slender fish, which isn't always the case. If wide-bodied fish will be kept in the tank, the calculation should be changed to one inch of fish for each twenty inches of surface area.
Like the one-inch rule, the surface area rule isn't perfect. Its primary advantage is that it takes into account unusually shaped aquariums.
Which Calculation to Use?
As a general yardstick for normal situations, the one-inch rule works adequately and is very easy to calculate. If using it, always use net gallons of water, and take into account the adult size as well as the shape of the fish.
If the aquarium is a non-standard size, the surface area rule will work better than the one-inch rule. In either case, always do your homework first, and err on the side of going under the limit rather than over.
Also do not fully stock the tank all at one time. No more than 25% of the total volume of fish should be introduced at one time. Fish wastes, which are toxic, are eliminated by colonies of beneficial bacteria.
Those bacterial colonies need time to adjust to changes in the bio-load. By introducing fish a few at a time, the bacterial colonies have sufficient time to grow and take care of the toxins produced by the fish waste.