I have had only two experiences with unwanted crabs, neither of them good. First one grew to over 5 cm before I ripped everything out of the tank to get him. In the mean time two cocoworms and several other tube worms disappeared, never to be seen again. I Just took a small crab out of my nano that was not invited as well. I knew something was up as every morning I would find some of my smaller corals face down in the sand bed. After a few weeks, I found one of my corals torn up pretty bad. This crab was only about 2 cm across. To me, not worth the risk, as identification is at times almost impossible as there are thousands of species that can be found in the tank, and I would suggest most are not beneficial. I thought about building a small tank, three or four gallons, just to put these little buggers in. Maintenance would be very minimal, but haven't got there yet
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