What is this?

[Chipmunk]

It started off with just a few but now I'm finding a lot of these little guys so they're definitely reproducing. Does anyone know what they are and if they're good/bad?

I'm attaching pics but these little snail looking things have 2 antennas, they have a long slug like body and the mouth is exactly like the turbo's I have in the tank. The small ones are probably 2mm x 4mm whereas the bigger guys are reaching 4mm by 10mm. They're mostly nocturnal.

I've never had a tank with these so I'm wondering if anyone else knows what they are?