olive snails good or bad

[Wazzel]

I recently found a few alive snails scurring around my tank and was wondering is anyone has had any experience with them. I do not have a pic and do not know if I could they move fast and are kind of small.

[achilles1]

I have had one for a couple years. I rarelt see him (once every couple months) as he burrows under the sand. I was going to buy more to stir the sand, but I think i read somewhere that they were bad so I backed off on buying more. Mine has never caused a problem and is quite beautiful when I see him.