Fish and shrimp getting in corner overflow box

[drcole]

like the title states, I have 2 small clowns and 2 peppermint shrimp that I found in my corner overflow box. How do I prevent this from happening? The only area that is large enough to allow this to happen is where the return line comes in

 

[sbash]

I had to block mine off with egg crate. However, new peppermint shrimp still fit through those holes, so I stopped buying them, unfortunately. I do have a large peppermint, which either does not fit, or he is uninterested in that part of the tank.