fish tank -- a dude's paradise

Today I asked a guy I recently met at work -- a guy about my age -- if he had a pet, and he told me he's had a large aquarium for many years. And went on to say a friend of his is a marine biologist, (?!!) who told him all his fish are males! Weird. I wonder why? Anyway, not one to miss an opportunity, I began singing ... (not the whole thing, just a few seconds) ...