No, they aren't predatory on fish. Maybe if it is dead, but not a perfectly healthy one. There are some species that can eat fish, but you don't see them often, unless you are unfortunate. As far as number and size, if you reduce the amount you feed, you can often curb their population substantially without resorting to the methods you used. I highly recommend replacing some of your rock with cured and/or established pieces to jump start things and add some more diversity that was lost as a result of the hot water bath. This is in addition to relocating the current residents.
