Whelks will eat other snails. They will also bite on fish that are asleep. I've lost a fish this way. Each night they would find him in the rock and each morning he would have more jagged flesh and bite marks. Get rid of them. I tossed mine outside. When I had them it was from an LFS that still sells them to this day. Reach in and grab them. If you kill the return pump and put some frozen food in the tank to blow around they will go hyper active and you can grab them.
Also Marc replied with a link to his write up when I posted a pic for a invert ID and someone spotted a whelk. This really goes to show how beneficial this community is.
