I fell into assumption I "needed" a clean up crew when I started.. Ended up I didn't need anything. I only had a mediocre diatom bloom that I blew off with a turkey baster and netted up with a cheese cloth type of net. After a week or so it went away. Wait for a problem to arrive before you jump into buying anything. To answer your question I had the most evil of all "HERMIT" crabs and Astrea snails. Get good quality cured live rock and your chances of needing anything will be greatly reduced.

When I first started up I followed the usual recommendations and added many different snails and crabs. But not because they were needed. Only because thats what I thought I was suppose to do. In general most of them didn't last long. The crabs would kill each other and some of the snails or get eaten themselves by my fish. The snails would get stuck in the rocks or fall upside down and die or just plain starve if I bought the wrong type. More and more I witnessed a hitch hiker cleanup crew emerge so I started to question what was the point in buying all of these things? So I stopped and now the bristle worms, mini brittle stars, mini turbos, pods, mysids, astrea stars and stomatella snails do most of the work. Whats left of my store-bought crews consists of 2 abalone, an astrea snail, 1 tuxedo and 1 rock boring urchin. Thats it and if you look in my Tank Journal you'll see that I do have a relatively large system with a high bioload.

+1, my reef won't ever be without hermits. I think I have about 9 different species in my tank.

One more question. Will they scare fish at night like sand sifter goby and wrasse during their sleep?

It's kind of like asking "does your dog bite?" maybe not today, maybe not tomorrow, but one day he will. Crabs are jackals, they will take what they can when ever they can get it. I hate crabs lol

yes they will when roaming around they might bump them or fall on them from the overhang.

Crabs and other crawlies help remove detrius from coral that is not able to do so on their own just by walking over them.
http://www.advancedaquarist.com/blog...ers-the-better

With all due respect isn't he talking about corals in the wild? I don't think we are in jeopardy of having our corals buried in sediment anytime soon. That being said we should all join together and have all crabs shipped back to the ocean where they can perform the important task of keeping our future corals safe.

Beside crabs eat dead fish what else eats dead fish?

would be easier to make of list of things that dont:P