Generally....unless you have my crab
I've been laying the glass right up against the rock he hides in and all I have caught are snails and hermits. I can even see him come most of the way out of the rock....just not into the glass. The problem is that the rock is on the very bottom of the tank and partially in the sand.
The faster you can get them out the better! I've lost fish to this guy. Lots of snails and even a fair number of zoas.
This is the same type of crab that I have: http://www.wetwebmedia.com/Arthropod...s/crab006a.jpg
My wife thinks I'm crazy when I'm by the tank with a LED
headlamp and a fondue skewer