First off I find it amazing to hear people say the only have 2 snails in a 180G aquarium. I must have 25-30. I do find the odd empty shell but have also seen a few hermits with an upgraded home due to those shells. As for the hermits I have found them to be killing off each other but not not in massive numbers.

I have 3 common and 2 tuxedo Urchins and have found them to knock over the occational coral that is not secured very well but usually just. We have three bumble bee snail and have neve seen them on a coral but I have had a few bad spots on a some toadstool as of late (could they be to blame?) I also have 2 fire shrimp (they are always cleaning the fish) and 2 peppermint shrimp(not an once of aptasia in the tank). I also have a blue star (only see him once a month) and an emerald crab (only see him once every 2-3 months) and the cow of my tank a sea hare. He is by far the the bull of the tank, he knocks over any and everything, I swear he rearranges my rock for me. We give him a small sheet of nori everyonce in a while to supplement his diet every once in a while but he has been in the tank for about 6 months without any hair algae.

I dont know if I would put in the bumble bee snails again but the rest of my clean up crew we love. I never (I mean NEVER) have to clean the back wall of the tank or the overflows, (if they look a little dirty I just drop a few snail in the over flow, and its clean in two days)