Thanks, B. You guys are too kind.
I've had those snails for some ten years on so, about as long as I've had fine sand bottom tanks. Got a fistfull of them from a LFS.
They are good breeders and really, they stir the bottom quite well, I've never had a bottom go bad, as fine sand sometimes does.
I'm thinking there's a bit too much of them in this tank and am thinking of an appropriate puffer to cut 'em down a bit.