I had some of the brown nassarius snails once and found out they were vicious. They attacked my garden eel and he had a terrible sore on his side. Once I removed them he healed nicely and hasn't had a problem since. I thought my eel was a gonner when I pulled the snail off him but I'm so glad he made it. He has lived a further three years so far so he wasn't a sick or dying fish when they attacked him. I purged my tank of all the brown nassarius and only buy the white ones now. I don't know if they are your snail killers but I still don't trust them.