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They sit on top of the sand, giving the crabs an obstacle to climb (albeit only 1" high) to get into their dishes. We don't see our crabs at all during the day, but I know the two smaller ones come out at night (have seen them climbing our cork wall at 2am). Since switching from clam shells (which I were told are too shallow) to these deeper water dishes, I haven't seen much evidence that the crabs are using them.
I see some sink their water dishes in the sand, so the top edge of the dish is at substrate level, which seems like a good idea... until you want to dump and change the water.
Advice?