lol I didnt even think of a shelf. Yeah you can just buy one of these from Super Store and suction cup it to the side of your tank. It would be pretty hard for a crab or any other predator (if that is in fact what is causing the decline) to get in there. Or you can buy gutter guard at pretty much any hardware store.
This is a shower basket, you'll be able to find them in the bathroom section of the store.
