Do they scatter and hide when you get near the tank or are they pretty bold and stay? You might try a bee trap model. Take a piece of tupperware/rubbermaid, cut a small hole in the lid. Place bait in inside with lid on and set in tank. Maybe one inside the other, with opposing holes. This would slow them down a bunch. They could get out but if you sneaked up on them you could plug the hole before they got out. Just an idea. Good luck.