Snail - approx 3mm - 4mm in diameter

[byee]

I let a friend put a bunch of his corals in my tank last summer because he was in the process of moving and needed to re-establish his 170 gal tank.

My tank is now 'infested' with these small little snails. Do not see many when the lights are on. There is literally 100's in my 120g tank.

Anyone have any ideas how I can get rid of them or just manage the situation by controlling the population ??

HELP !