I know I had a 6" pleco in my female betta tank. He didn't bother them at all. He just kept to himself. Every once in a while he'd grab one of the snails and give them a nasty wake up call by trying to clean off their shells. Sadly my tank was over run with something and I lost all 15 snails in less then 24 hours, the pleco two days later and one of my female betta's a few days after that. My male betta is fine and I still have two female betta's who are fine. Don't know what happened.