Some people will tell you 1 snail per gallon. I disagree. I have a 120 that is doing great with 6 or 7 super nassarius, a few trochus, a few cerith, ummm, i think thats it for the big tank. All together about 15-20 snails. The Super nassarius are great at finding uneaten food bits and stirring the sand bed. I think a lot of snails will eat detritus, but not if they have other options. The prefer to eat other things.