If you have trouble getting a fish trap, you can make one out of a 2 or 3 liter plastic soda bottle. Wash it REAL GOOD!, cut the top end off, where it shrinks down to the nozzle. Cut the nozzle off, and make a hole just big enough for the Damsel to swim into, turn the top over and put it back upside down. Then you need to epoxy it back together with non-toxic glue or silicone.
You might need to leave it in for a little while so the fish get used to it being in there. Then squirt some food into it with the fish watching, they will figure out how to swim into it... the only problem is getting the right fish to swim into it! When you finally catch it in the bottle, you should probably place a net over the hole, just in case, but usually fish don't figure it out very easily.
You might have to make several attempts, but it beats pulling all your rock out.
Also, you probably want to put it on its side.
