Easy. Wait until a few hours after lights out, make sure that all lights nearby are out as well to keep it pitch black. Turn on the lights suddenly and the fish will be disoriented and very easy to scoop up. The faster you get in there the better. Usually u have about a minute.
build a fish trap out of a plastic bottle. put some of the fish's favorite food in the trap. attach some mono-filament to the bottle so you can pull it out once the fish is inside.
it works. be patient. use a long piece of mono-filament so you can sit down and relax somewhere not standing right in front of the tank.
Best approach is to drain down all the water until he's flopping on the bottom and grab him. If that seems too extreme, then night time ambush is worth a shot. If that doesn't work then a fish trap is next up. I have used the barbless hook, but it'd not be my first approach.
You can dump him into the sump or quarantine tank on the idea that you remove the fish one at a time until you catch your target. Or just release him back into the display tank until the target fish wanders in.
Just sneaked at my poor little angel and netted him up. Hes gonna go into brook treatment with my other 2 remaining fish a percula clown and a hifin goby. After 6 months in the hobby i have a thriving coral reef but lost 75% of my fish to what i now know mustve been brook.
So, I bought an expensive fish trap ($60) and APPARENTLY, all my fish (clowns, blue tang, and longnose hawkfish) are all more intelligent than both the fish trap designer and their owner. I tried baiting it with various types of food and not one of the fish will go in, after many days of trying. Just my 2 cents of opinion!
Trap has to be in place not touched for 2 to 6 days. Bait the trap and cut back feeding to what they eat in 45 seconds. It works but needs time until they feel it's part of the place. My 2 cents