Just keep buying a few everyother week or so till they even out with your tank. Every tank will be different because of different bioloads and different animals kept. In my 30 I have about 10 hermits, 4 astreas, 1 brittle star, 1 turbo snail, 1 margarita snail, 1 emerald crab, and a few stomatella snails, and they keep up with the mess just perfect. You can add what you want but should add sand sifters, algae eaters, and poop eaters, and just enough to keep up with all the mess. You dont need hermits and they can be mean. I love them but some people run very sucsessful tanks without them at all.