overflow trouble!!

So i have a internal overflow with a stand pipe.
Well i have had some small fish get into the overflow.
Is there any way i can block the over flow but not restrict the flow?
Any idea will be awesome.

 

Many people use a plastic mesh like found here:
Plastic Canvas 7 Mesh | Hobby Lobby

My internal overflow came with a fitted top that keeps light out and prevents jumpers from hopping over, small fish could still get through the overflow teeth though if they were skinny enough.

 

The mesh noted above should work for you. in the mean while getting the little buggers out of the over flow can be a real pain in the backside. In the past I put a net under the drain pipe in the sump and then pulled the stand and let the overflow flush the little buggers