It took several days to catch my lone kuhli fish, Sammy.
So I tried making the opening smaller. First I stretched tight a small piece of plastic wrap over the hole with a rubber band, leaving just a narrow horizontal gap at the bottom. It defeated the platies- they would bump their noses, give up. Oliver was determined. He figured out how to bang the plastic with his nose until it gave some, they lay his body sideways and squeeze through. I was surprised he made it in, and impressed at his problem-solving skills! The kuhli almost came in at one point; could definitely smell the bait and wiggled up searching for the opening, got his head and part of the body in, then backed out and wouldn't return. I wonder if he didn't like the feel of plastic edge on his back?
Then I put a new obstacle on it. I got a stiffer piece of clear plastic, cut it to fit the opening with a flap on the sides, used a hole punch to make a small oblong hole, fastened it on tight with rubber band again (over the folded flaps). It kept Oliver out. He couldn't force his head in. I watched him try a few times, then he left and didn't come back to the trap. Smart fish. I left too, stayed out of the room for over an hour, confident now that Oliver wouldn't get stuck in there and I could leave it as long as it took for the shy kuhli to get hungry.
Later in the morning I sent my three-year-old into the room: "go see if there's something new in my fish tank." She went, looked, yelled excitedly: "Mommy! Sammy is in the fish trap!" Ha ha. Gotcha.
Now he's in the big tank with the other kuhlis. I've seen him hanging out with them under the log already, but he also seems to like the little rock cave I once made, crams himself in there repeatedly. I hope he feels at home and likes having some buddies.