romunov covered a lot of it, but I thought I would add my .02:
I would avoid the sand sifting sea star, it will starve in a 55 or 75 gallon tank. They shouldn't be kept in less than a 120 and probably not even kept at all. How do I know? I starved mine in my 55 7 years ago and then read up on them.
I would avoid all astrea or "turbo" snails and margarita snails unless you know they come from warm water. Most are cold water species and slowly die in our tanks. A LFS employee telling you he/she is SURE they come from warm water doesn't count
Astreas from Florida are usually pretty good.
The problem with advice on snails is that each species has somewhat different dietary requirements. So while a "nerite snail will feed on filamentous and film-forming cyanobacteria, and filamentous green algae" - other snails may prefer not to (the full article on nerites by j. sprung is linked below ).
Nassarius Snails for cleaning up excess food:
http://web.archive.org/web/20010410...ibrary/articleview2.asp?Section=&RecordNo=166
Nerite Snails:
http://www.advancedaquarist.com/issues/sept2003/invert.htm
Lots of info huh?
Basically:
Best:
Stomatella
Collonista
Trochus (warm water species)
Cerith
Nerite
OK:
Astrea (warm water species only)
Nassarius (only good at cleaning up excess food, you don't need too many)
Bad:
cold water Astrea
margarita
big turbos (knock over corals and even rocks)
sand sifting sea stars
Bumblebee snails (predator on some clams and snails, won't help with algae at all)
Feel free to ask any questions. I've also heard that hermits will eventually kill a DSB, snails, irritate corals, etc. I'm not fully convinced of this, but I only have a couple in my sump now. With the wide range of good snails above, I'm going to try no hermits.
B.