Stocking Density, also known as the Carrying Capacity, is determining how many fish will fit in an aquarium and is one of the most controversial topics in fishkeeping. There are two common rules-of-thumb, they are "1 inch of adult fish per gallon" and "12 square inches of surface area per 1 inch of adult fish". The problem with both of these is that they treat all fish the same. The bio-load and oxygen requirements of one 4-inch fish is more than the requirements of two 2-inch fish. Most aquarium keepers agree that stocking can be broken down to one question, will the fish have enough oxygen to breathe? Fish respiration will breathe in oxygen and breathe out carbon dioxide. A tank with low oxygen will have fish gasping near the water surface.
Only after a tank has been cycled should you add one or two fish every 3 days and stocking should be oxygen based. Oxygenation is the process by which oxygen in the air enters the tank as dissolved oxygen. This is done only at the water's surface where carbon dioxide is exchanged for oxygen. This exchange is increased if the water's surface is agitated, such as by bursting air bubbles or running water from a power filter.
Surface area is the tank's lengh multiplied by the width and the amount a tropical fish will need can be judged by it's adult body size, not including the tail. Since all tanks are different this formula is meant to give you a healthy starting point.
F = Adult Fish Length in inches S = Surface Area Required in inches
S = (2+2F)F
You simply take 2 and add the adult fish inch length multiplied by 2 and that is the surface area per inch it will require. Therefore, a 4-inch fish would have: 2 + 4(2) = 10, 10 is the surface area per inch it needs. It is 4 inches long and 4 x 10 = 40 so the 4-inch fish will require 40 square inches of surface area allocated to it.
Remember, an understocked aquarium is a healthy aquarium.
There are various types of fish foods available. To ensure freshness all dry foods should be replaced after 6 months of purchase and do not get the food wet prior to feeding. The fish should be provided with a varied diet and you should research their food requirements.
The type of food a fish will eat can be reasonably judged by looking at the position of its mouth. Fish with mouths pointed upwards are top feeders and like surface level food, fish with horizontal mouths will eat floating or sunken food, and fish with mouths pointing downwards are bottom feeders and need food resting on the aquarium floor. It's a good idea to first put the desired amount of food in your hand then drop it into the water to avoid a large spill from the container. Here are the standard food types:
Flakes: These are the most popular and basic. Mostly top feeding fish will eat them, but since they sink after floating on the surface for a while even bottom feeding fish will have a chance to consume them. There are also flakes available for specific functions like color enhancement.
Tablets, Wafers and Pellets: These are designed to either float first then sink or sink right away. The tablets may be pushed up against the side of the aquarium to give top and middle feeding level fish a chance to eat. Tablets or wafers are the best way to feed bottom feeders. Be sure to look for ones that will not cloud the water.
Freeze Dried: The two types above are made from cooked ingredients, freeze dried food has aquatic insect larvae and crustaceans. These are the most like the food fish have in the wild and common types are bloodworms, tubiflex worms, and shrimp.
Frozen Foods: Frozen mini-cubes of bloodworms, brine shrimp, and tubiflex worms are also available.
Live Foods: Almost all fish love these since it's what they eat in the wild.
You may want to turn off the power filter for the few minutes that the flakes are floating in the water or they may be filtered out before the fish can eat them. Those flakes will go into the filter's media and begin to decay and contribute to the water pollution.
Important: Do not overfeed. It's not healthy for the fish and will quicken the pollution of the tank. The fish can be fed once a day, and only the quantity of fish flakes that they will consume in 3 to 5 minutes. Bottom feeders should be given just enough sinking wafers to consume in 1 hour. If you skip a day, that's fine. Fish can go days without eating and the ones in the wild often do.