Finding my Goby in my corner overflow and sump

[Jimbo Bucket]

I keep finding one of my Yellow Clown Gobies in my corner overflow or washed down into the sump.
I don't know if he is hiding from other tank mates or just gets accidentally washed in.
What are the thoughts on filling the gap between my corner overflow and return nozzle pumping?
There appears to be significant flow through that crescent.
I'm scared to just block if off willy nilly and overflow the tank...... I've never tested it, but I would guess my return pump section of the sump would run dry before i overflowed the tank.
What have others tried with success or failure?

 

[Reefnewb]

Theyre a fish that likes slow flow. maybe he tried to escape the flow and got sucked in by accident. You could put some plstic mesh infront of the teeth of the overflow but you would want to make sure nothing blocks it frequently!

 

[Ridem32]

Use some type of screen that fish can't get thru but water can still flow thru.