The trick I've used several times to get fish out of my overflow. It'll depend on your plumbing if it's an option.
I have a herbie so there are three overflows going to the sump. On one of them, I have two unions between the tank and the sump. The unions are below a shut off valve. I shut off the drain, take out the section of pipe between the two unions, then I've made up a short pvc section of pipe that'll attatch to the top union, so when I upen up the vavle, the overflow will drain out this short section of pipe into a bucket on the floor.
To catch the fish, I guide the fish over by the pipe, wait until it's close and quickly open the valve. Once the fish disappears down the slip and slide into the bucket I shut off the valve. Fish in a bucket