First off, I'm new here, so I don't know what the acronyms FOWLR and LS, so my comments may not be apropo, but here goes. I had a small horshoe crab
(Limnus sp.) about the same diam. as a golf ball in my reef tank. It did an /amazing/ job of keeping the sand clean and "mowed" looking. It never
bothered any of it's tank mates and vice versa. It was also very furtive and I wouldn't see it for weeks at a time and never climbed on the rocks or
coral, it would always remain buried and occasionally come out to change locations.. You just need to make sure you're using a reasonably fine
substrate (I was using fine aragonite) and that it's deep enough for the crab to bury itself completely. As for fiddler crabs, if you're referring to
those of the Uca genus, then they would only be suitable in something like a paludarium since they spend most of their time on land (especially mud)
or burrows therein. I suppose they would live in an entirely aquatic environment but they probably wouldn't be happy. I had some cuttlefish awhile
back and to save on money I'd go down to the local mudflats and collect fiddler crabs to feed them. The uneaten crabs would survive fully submerged in
the tank for quite some time and seemed to be doing ok, but I'm talking only a week or two here.

What do you all think about Horseshoe Crabs and Fiddler Crabs and FOWLR and
LS ??

are they ok or should i stay away from them ??