Flyguy is the overflow wide so the flow will be shallow and slower? If so you will not loose many fish. If when overflowing the water is fast moving and deep then yes you will loose some each time. This has been my experience anyway. If you try screen it off with wire or fencing you will find it will fill up with debris and cause new problems. I have an 18" culvert overflow I found when that pipe gets near full it sucks through a lot of fish. I put a 55 gallon drum over it with 1 1/2" and 2" holes all over it. This slows the flow and seems to save my fish. At least that what I tell myself. It does of course take longer to drain down back to normal level.