Fish may grow on the outside in proportion to the size of the tank, but their insides continue to grow. This is very agonizing and painful, and will cause premature death. You should always have the proper size tank for the adult size of the fish from the very beginning. Some folks like to grow out their fish in smaller tanks, but then you leave it to chance, and circumstances can arise where your plans of getting that bigger tank don't happen.kerryve said:anyone correct me if im wrong but if you buy a common pleco as a baby he will grow in proportion to the size of you tank, ive had a few plecos that have not grown too big for their tank, but ive always ended up getting bigger and bigger tanks so they were never able to out grow their smaller tanks. otherwise there are some largeish snails outhere that one could get.