I had fish and snail in the overflow all the time.
I used plastic gutter gaurd from Lowes or Home Depot. Cut some larger than the top of the overflow, bent it to shape and used cable ties to keep it in place. I haven't had anything in the overflow since.
Not my idea, got it from people here. I bought a whole roll. If you want some, PM me with your address and I'll cut off a foot or so and stick in the mail.
Dave