When I did my shrimp trap I put some rocks in the inverted top -- big enough so they wouldn't fall in the hole, but they helped weight it down and kept some of the fish out.
Julie
37g planted, 11g planted, and three 5.2g planted shrimp tanks.
It's the same principle as the commercial fish traps my nieghbor uses, except the mouth is on the side instead of the top, it's funny they just can't figure out to swim up the middle.
