+1 to hank, I’ll add that I let the tank tell me if it’s stocked or not. In my 20 cube I have 4 fish and am thinking of adding a 5th as my nitrates are consistent in the 3-5
ppm range and I’d like that a little higher. That said I’ve chosen size appropriate fish that inhabit different regions of the tank, a shrimp goby for the bottom, pair of clowns, and a tail spot blenny that perches in the rock work. For my 5th I’m thinking something like a 6 line or orchid dotttyback or gramma as they’ll occupy different territory than my current fish