40 gal tank, 3' long, 18" deep and high, how many small fish can be kept in there? I have a bta, a cc star, 2 Occellaris clowns and 2 chromis, 4 small snails. Am I overstocked? Can I put in a few more snails or hermit crabs, to keep the sand stirred up and clean? I am getting brown spots on my...