I have 3 candy platies (adult size 2.5 in), 3 tiger barbs (2.5 ") , 4 zebra danios (2") , 2 pearl gourami (4") , 2 albino bristlenose plecos (5") , and two red swordtails(4") . I got the number by adding all the adult sizes and going by the rule of thumb for one inch per gallon.