they say its 1 inch per gallon, but if u do small schooling fish u could 2 inches per gallon, and for larger fish 3-4 inches per gallon. the phrase "u can't possibly keep a 10-inch pleco in a 10 gallon tank, but u can keep definatly keep more than 5 cardinal tetras in a 10-gallon tank." is commonly used to discuss this factor.