I"ve been moving some stuff around in my aquarium lately and have spotted at least 3 crabs that made it past my dips when I got LR. I want to build a trap and bait it with krill but I think my shrimp will just swim in and take it if I just lean a glass against a rock. Do you think the soda bottle trick will work and do I actually need to place it against the rock I saw the crab in or will they find it eventually?