Planted Tank Guru
Join Date: Jan 2010
Location: Contra Costa CA
There is a very old formula that suggests 1" of fish per gallon of water.
This can be the beginnings of a guide as long as you remember:
1) It only works for fish to 2" long (not including tail). Do you cannot figure your SAE this way.
2) It only relates to the chemical needs like O2 supply and CO2 and ammonia dilution.
3) It says nothing about the social needs of the fish. Do research aimed at these areas:
a) Aggressive fish for territorial or other reasons.
b) Social needs of schooling fish, or fish that may try to breed in the tank.
c) Size ratio of the largest fish you want to the smallest: Most fish, even if they are not strictly predators are willing to eat a fish that is up to 1/4 of their own length, and predatory fish are often capable of eating a fish that is up to 1/2 their length.
d) Activity level. Average fish should be in a tank that is at least 10x their length. A 4' tank might be OK for fish to 5".
4) Assess the tank and the aquascape and research the fish: Fish that thrive in very fast moving water are not compatible with a tank that is stuffed full of plants. Cool water natives are not compatible with warm water tropical plants. Hard water fish are not compatible with that short list of plants that must have soft water. Fish that dig and burrow into the substrate are going to be pretty hard on rooted plants and so on.
5) The more nitrogen you add to the tank (protein in fish food is the primary source) the better the plants need to grow to remove it. This directly relates to stocking levels, light and CO2. More fish = better conditions for the plants, or else you will be doing more and larger water changes to keep the NO3 in a safe range for the fish.