I'm planning on adding either shrimp, snails or both into a 20gal Fish tank, which right now contains only a male betta, along with some other fish. I'm also going to buy a pre-filter sponge so that nothing gets sucked up into my power filter. What I'm looking for is not only the pros and cons of each option, I'm also something that would not only try to escape my tank, but also won't try to eat my pre-filter which would turn my power filter into some kind of vacuum cleaner. Any suggestions? Right now I'm thinking of assassin snails and ghost shrimp, but only if they are a good mix, other wise it would one of the two.