New to reefing.

[Myka]

They do add to your bioload, but not nearly as much as a fish does. There isn't really a rule. The 1 per gallon thing is a joke...they'll starve to death. Start with a couple of each type you want to try and go from there. My Astreas have been very hardy. I bought three when I first set up my 33g in Dec 06, and they're all still going strong. They are an uncommon species called Astrea caelata. To give you an idea, I have 14 snails in my 33g. I also have an Orange Lip Conch. I find my Astreas, Margaritas, and Conch do the most work. The Nassarius population rotates due to them dying off (maybe 50% per year), Cerith as well although at a lower rate (maybe 25% per year). I don't find the Nassarius do a whole lot, other than picking at uneaten fish food, but my Peppermint shrimp does just as a good a job. The Nassarius also churn the sand a bit, but I find my Conch does 10x better job of that! I won't replace my Nassarius or Cerith in the future. So here's a breakdown for you:

Orange Lip Conch - churns the sand, keeps the sand clean, cleans the lower 1 1/2" of the glass (as far as he can reach haha)

Margaritas & Astreas - I think do pretty much the same thing, which the Margaritas being a bit more efficient. They do a good job of the green & brown algaes on the glass and rocks.

Cerith - stick to the glass, and don't do a whole lot.

Nassarius - stick to the sand, and due their small size, don't do a whole lot. They do churn the sand a bit, and eat some fish food, but other critters do a better job imo.

Stomatellas - awesome algae eaters, and will reproduce in the tank. My favourites.

Peppermint shrimp - Pretty much attack uneaten fish food. Good scavengers. Often eat small aiptasia, but also ate my Stomatella snails.

Cleaner shrimp - Good scavengers, will also clean some fish of parasites.