Removing fish from an established display tank can be very difficult!
The first thing to remember is that the fish are much faster than you so don’t try to catch them with speed, this will only stress them and you. The first thing to do is add a bunch of large nets to the tank and don’t attempt to catch the fish. Leave the nets sitting in the tank until the fish get used to them and a not afraid of them. Nets can be bought cheaply and are worth having a few of because they will make your life easy.
After you leave the nets in overnight hold your hand on a net and feed the fish in front of the net. When all the fish swim up to eat the food scoop all the fish from behind. Aim to catch all the fish don’t let the fish know that you are after it or it will stay in the rocks when it sees you. Once you scoop all the fish pick the one that you want out of the net and release the rest.
If you miss it don’t try again but leave the nets in the tank and wait till all the fish are asleep. Then locate where the fish sleeps and very slowly prod the fish into the net using your hand or the other end of a net. Move very slowly so you don’t wake the fish up.
If this is not possible to get it that way get a barbless hook attached directly to a sinker and slide it down a bit of conduit pipe. When the fish you want comes over and is interested in the baited hook lower it out of the pipe. If other fish go for the pipe pull the hook back into the pipe so other fish can’t get it. Once you hook the fish pull it out and remove it straight away. A correctly sized hook will create very little damage to the fish.
You can also get a coke bottle and cut the top off it and jam it back inside its self to create a fish trap. Add a bit of food to the bottom of the trap and release and fish that you catch each morning if it is not the one that you want. There is also some traps that you can buy which work quite well.
As a worst case scenario if nothing is working, you may have to strip the tank of decorations to remove the fish. This effort is worth it if the fish is creating a problem to you.
Good luck and enjoy,