I think they are baby turbo snails...???

[vmalpi]

I found these on my filter when doing maint on it.

 

[Fluff]

That is a baby limpet.

 

[vmalpi]

Can i put them back in my tank? are they good to have, bad?

 

[Devilishturtles]

Good algae grazers, I'd put them back in.

 

[MT79]

I have tons that look very similar to those. Like DTurtles said, free tank cleaners! Their numbers seem to rise and fall, I assume according to the amount of food available. Theye've never caused problems for me, but FWIW some limpets may cause issues, ex. keyhole limpets.