He's goin back in
 

Thanks for all the info, I am going to put this guy back into my tank as I really can't see how he could eat fish. He is quite interesting to look at, wonder if I'll see him again. If the slug website identifies him I'll let you know what he really is.

be careful with nudibranches. Many of them are highly toxic when they die. Could kill your tank if/when it dies.