The bacterial count rises, but only so fast: they have to eat before dividing. Add too many fish producing poo too fast and if the bacteria aren't enough---ammonia happens and the fish die. My recommendation is to start out for a few weeks (past cycle) with some invertebrates like worms, snails, micro-hermits, and let them help the bacterial count grow. [You do feed them.] They can 'break in' the tank safely, so that by the time you add one fish to the mix, there's no problem at all. Also, quarantine your first fish while you're watching the crabs' antics, and you'll not be introducing parasites into the tank---you cannot medicate a marine tank: it's 72 days of no-fish if you get a parasitic infestation. So slow and careful will save you a lot of grief.