i got some drift wood at the fish store today and i had the girl there knock off those little black snail's and when i put it in my tank two little black snail's fell off on to the floor and did not know if they were a hazardous to my tank i heard a long time back that the reproduce very fast and a a- sexual and they can take over your tank i just need some knowledge about them and if i should just take them out ? pleas help