Best way is going to probably be to shut off your return pump, and siphon out most of the water in your overflow, and go in with a small net and catch him. If you have a net long enough.
Long fishnet and another long stick (plastic or wooden one,).
Put the fish net down and with another stick try to drive in the fish to the fishnet.
A lots of time did it in my overflow box. my box size is 4*8"*24"H. ))
Still no luck here! The problem with draining is that it would basically require me to empty the whole tank and I am not sure I am willing do do that. I tried draining ~80 gal and he still had plenty of water to swim away from me in. I also can not adjust the pvc plumbing in the backX Further! I gave up after at least an hour of trying to get him and went to check on him just now and he got sucked down into my sump. Still alive, but now I dont know how to get him from here.... I cant pull the pvc pipes out due to clearance btwn them and the tank.
Let stop the return pump. Sump will be filled with water. Add some water to make water level in the sump higher than compartment with the drain pipes. Put some fish food flakes on the water surface and wait when the fish swim in to another compartment. Then catch him, or switch the return pump on.
Got him! I turned the system off and tried siphoning the water out of that compartment but was still unable to get to him once the water was low. I cut the system back on and as that compartment refilled I was able to move one of the pvc pipes around enough to get him towards surface and he was pushed into the other compartment. Netted him, back in the tank. He is a little beat up from his adventure but I think he will be fine. Hopefully it wont happen again!