Catching a single nuisance fish in a fully stocked aquarium can be tough. The one you want to remove is far too smart to swim into a net, and as soon as you approach the tank it darts into the rocks. Thinking a fish trap will work? Good luck. You’ll end up catching everything but the fish you’re after. To turn the tables a little, scare the fish into its rock of choice. Once it gets settled in for the long waiting period until you give up, reach into the aquarium and pull the entire rock out. If the fish is really deep into the rock, you can set the rock onto the ground and get after it with a hammer and screwdriver. After a few whacks of a hammer, there won’t be anymore crevices in which the fish will be able to hide. That’ll show him.