The best way to catch a fish without a fishtrap is pretty simple, just wait until they fall asleep lol. Keep the room the tank is located pitch dark, then suddenly turn on a bright flashlight (UV flashlight seems to work really well also). This stuns them for a good second or two while they're out in the open, which should give you enough time to scoop them up. Have a big net and a long stick ready, sometimes you have to use stick to steer them out of the rocks and towards the net. I caught some of the smartest fishes this way, I caught them lol. Just wait about 3 hours after the lights go off when they're deep asleep