I have had this problem before, but not with the larger gold fish and Koi. It has only been the smaller, 2"-4" that get in there. The wier door floats when the skimmer pump is turned off, so I don't think the fish could escape without water flowing over it. Usually the fish can swim over it, but we had some pretty stiff winds that knocked a lot of leaves out of the trees and into the pond yesterday. Our smallest Koi got in there and was buried with leaves in the basket and died. Problem is, everything I think of to keep the fish out will also keep the leaves out, which pretty much defeats the purpose.