In the past we have had some fish get in the skimmer, luckily there is a net on the inside so they are safe as long as they are strong enough to keep swimming, the poor frogs jump in and get on the wrong side of the net and past the pads and end up as bits and peices in the biofalls. We have used numerous methods, putting a large weaved screen on is fine if you don't have lots of leaves, or debries that floats in the pond, you just have to make sure to rinse the algae off occassionaly. Have also tried a milk crate with smaller openings.....same thing. Problem with most blockades is the floating debris, you don't want to block the weir door to much especially if you have a pump in your skimmer.