Coming to this one late, so this is for anyone who lands here with the same problem. The trap advice above is sound — I've used plenty of them — but the part nobody warns you about is the wait. Most need several days before the fish is comfortable enough to go in, and someone in this thread spent three months before giving up and pulling the rock apart. When you're trying to get an aggressive fish out before it does real damage, days matter.
Full disclosure — that waiting game is what got me building the
AquaCove (founder of FITC Aquatics). It's a dark cave you corral the fish into rather than baiting it and waiting it out, so it works off the fish wanting shelter instead of needing it to trust food first. Doesn't replace the methods above, but it tends to move faster.
