Question About Tank Cycling Time

[MMAX]

There's quite a bit of coraline and hair algae on the rocks which the crabs and snails have been taking care of. I recently noticed quite a few copepods running around and some weird kind of slug or snail (which I didn't buy) crawled out of one of the holes in the rock. I've had maybe 3 crabs die so far and didn't remove the carcasses hoping that would help. Maybe I'll try the fish. Should I be feeding the crabs also or can they just survive on algae on the rocks?