It's easy.
Cut the top off a 1 liter soda bottle. Use a steak knife. We're not building a space shuttle here, just hack it off.
Turn it upside down and stuff it back in the body of the bottle to trial fit. Cut a little off the neck of the top if you need, so it will slip inside the body of the bottle.
Run a big goober of 100% si8licone around the top part where it will contact the body of the trap. Stuff it in and let it sit for 3 or 4 days to cure.
Starve the fish for a day or 2. Put a couple rocks in the trap so it will sink. Put some food in the trap. Drop it in the tank. Don't feed the tank. If they want to eat, they go in the bottle.
If they refuse to eat, take the bottle out after about an hour. Drain it by poking a hole in the bottom. You can poke a large hole, so long as the dottyback can't fit through the hole.
Drain the food and water out. Leave the rocks inside. Stick it back in the tank so the fish get used to it. Tomorrow, try it again. No food in the tank. They swim in the bottle to eat or they starve.
You'll catch him eventually.
Removal? Steak knife. :mrgreen: