There are many fish traps out there as mouse traps, but fish learn and are smart.
My neighbor and I tried to catch his 4" blue damsel for 2 months in his 180g. What eventually worked was a large Seaside Aquatics trap. It figured out every trap we used. What seems to work best is traps that have a sliding door, a small hole to squirt food, long fishing line to control the door, and patience. Took 4 days but we finally caught it with it's favorite food, PE Mysis.
The damsel = credit at the fish store for... more PE Mysis. Good luck.