Set up my first tank yesterday, in preparation for getting my tropical fish in a few weeks time. No fish in there yet; just gravel, ornaments, filter, heater, fake plants, and a couple of live ones. It is tiny, with small antennae on its head. Have just gone in for a look, and I now can't see it, but it had been sort of swimming around near the glass at the front of the tank. Not sure if it actually had a shell, now I come to think of it, but it probably did. What to do? Leave it (I've heard they can multiply and take over) or entice it and the family out using the lettuce leaf method? Weird ... I had washed and examined the live plants quite thoroughly prior to putting them in, but no sign. Thanks in advance.