well, i had the same problem about 3 months ago. fish kept dissappearing, and i could never find them. well, i ended up spotting the culprit one day. i never could have anyone tell me exactly what type of crab it was, all they could come up with was a rock crab. he was a mean
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though. and very nocturnal. he would even hide out at night when i had my small red flashlight on him. he also had like a cave. he broke out a big hole in my LR, climbed in, and held the rock he broke out over the hole like a door...very cool. so search your tank for something hiding out in LR. one things for sure, he was easy to catch. setup a trap, and he was in it within 20 minutes. haha! here is a link to the pics of him. he was very similar to an emerald crab (shape and size wise), but had MASSIVE pinchers. check it out:
<a href="http://www.imagestation.com/album/?id=4291634997" target="_blank">http://www.imagestation.com/album/?id=4291634997</a>
notice in the pics....he was picking up an abandoned astrea snail shell, and carry it back to his hole to block it. very interesting and smart crab. good luck with catching him...any questions just let me know.
-One