I need to learn to control the numbers, i want them there as its sand substrate and they keep it turned for me.. and they are really cute

The 4th big one down in the pic is the mother, grandmother, great grandmother etc. of every single snail there!

He/she came in the bag with a bristlenose plec i bought nearly a year ago, i plopped her in a rubbish thing on the windowsil one day with plant trimmings.. and found her and seven more alive about 6 months later (i didnt bother cleaning the thingy for ages) so in they went to stirr my sand, for ages whenever i counted there were only 8 and i was thinking "whats wrong with them, why arent they breeding.. maybe i need more" then one day there were hundreds! ..phew that was quite a ramble.. i'll shut up.