are Malaysian Trumpet Snails. They burrow in the substrate and will mix it around for you. They don't come out too often except for mainly at night. Sometimes they are pests, but as long as you don't overfeed, I've never seen them as pests. In fact, I am going to get more beacuse they are cute.
Just pop in maybe 4 into the 5 gal
, maybe 6 into the 10, and 10 into the 20 gal
. Some fish will see them as snacks and eat them. But normally you don't see them at all except for maybe the tip of the shell poking out of the sand. I got mine for free from the lfs
. Most places just give them away.
Here's a pic