Am wanting to get snails and crabs for my fish only tank. Is the rule of thumb for snails (1 for every 2 gallons) for just one kind of snail, or the total for all of them. How many should I get for the sand (the Nassarius ones) versus how many should I get that eat algae off of live rock and glass? I have a 55 gallon. I also plan to get 1 or 2 peppermint shrimp and already have 4 hermit crabs. Is that enough crabs or how many? Thanks!