Originally Posted by blue_gourami45
a general rule of thumb is one inch of fish per gallon of water in the aquarium
That is no longer applicable. It is now considered a myth.
The needs of the fish or animals should be taken into consideration. These needs may include: Proper room for the Activity Level and Swimming Space
-Swimming levels/niches, the animalÃ¢â‚¬â„¢s usual style or level of activity such as Ã¢â‚¬ËœlapsÃ¢â‚¬â„¢ around the aquarium, schooling, and skittish behavior common in schooling fish.
*With schooling fish, the length of the tank is very important as in the wild, they shoal in long distances(pretty much applicable to fish with slim body shape and fish from river rapids). Territorial requirements
Ã¢â‚¬â€œthe floor space or empty room between individuals. Vertical space
-most taller-bodied fish require room above and below their fins.(most applicable to discus and angelfish) Compatibility issues
-many aquarium inhabitants will not be able to live together properly to the end of their natural life spans.
As for the different levels occupied by fish, this consists of:
Surface level-fish with upturned mouths(usually hatchetfish and butterfly fish), pencilfish
Mid level-fish with mouths forward(applicable to angelfish and discus)
Bottom level-generally catfish and loaches although most cichlids tend to occupy the bottom like the Tanganyikan shell dwellers
For the stocking levels, for example in a 30 gallons tank, there are 10 kuhli loaches, 15 harlequin rasboras and 15 hatchetfish. We do not consider this as overstocked as different fish occupy different levels of the tank.