Here's a rough schematic of the trap. There was a gauze bag of algae pellets inside. It caught quite a lot of snails but I actually collected just as many by scraping them off the glass into a net when they all came out to play after the trap was in place. I guess that the algae disperses into the water to some extent and they go exploring!
It worked really well over one night but the second night didn't yield many victims. Whether this was because the snails weren't hungry any more or not I don't know. My Khuli loaches took up residence in the flowerpot on the first night but it didn't seem to put the snails off too much!
Syd.
P.S. Just discovered that I can't upload images, so I've put the picture as both my avatar and my personal photo, I hope you can see them ok.