How big is your tank. I used 4 Mexican Turbo in a 125g with hair algae. It took some time (weeks) but they did the job.
Here's my snail info that I gathered over time:
Nassarius Snails - Nassarius spp. They mostly stay in the sand, but will sometimes make short trips up the glass. They are mainly detritus eaters and do an excellent job cleaning and shifting the sand. These are also some of the most hardy snails available. They can flip themselves upright very easily. They are fun to watch as they come bursting out of the sand bed at feeding time.
Onyx Nassarius -Ilyanassa spp. (Black Mud Snail, Black Nassarius Snail ) snails are very good at keeping sand beds completely clean of algae as well as other organics. They may occasionally strip a sand bed of enough nutrients that there will be none left to support copepod or amphipod populations. If you keep dragonets (e.g. Mandarins) that rely on healthy populations of copepods and amphipods do not get these. They are also a cooler water species and may not last that long in higer temp tanks like reef tanks.
Cerith Snails-Cerithium spp. (Cortez) - Good algae and detritus eaters that forage rock, glass, and sand. Some can pick themselves up and some cant.
Cerith Snails (White)- Good algae and detritus eaters that seem to stay in the sand more than the cortez, but can be found on the rock and glass. Once again, some can pick themselves up and others cant. These are good sand bed snails.
Fighting Conch Snails - These guys are all about the sand bed and are very cool to watch. They usually dont get flipped over, but if they do, they can kick around and turn themselves back upright.They very hardy too.
Astrea Snails -Astraea tecta: Most common of all saltwater tank snails. They are excellent algae eaters and will forage all over the rock, sand, and glass. These guys fall very easily, can not right themselves, and then die.
Margarita Snail - Margarites pupillus. (Stomatella Limpet Snail, Pearl Snail, Little Margarite Snail, Pearly Topped Snail ) Another snail to add to the algae eating aresnal. Will cruise around on the rock and glass.
Fighting Conchs - Strombus gibberulus. Little vacuum cleaners. One per 2 sq. ft. of sand. They'll eat diatoms and, sometimes, cyano
. Get one per 2 sq. feet of tank is what was recommened to me. They tend to disappear behind the rocks for a few days then come back around to the front again. They usually dont get flipped over, but if they do, they can kick around and turn themselves back upright.They very hardy too.
Nerite Snails - Nerita spp. Mostly a rock and glass snail that are all about the algae. They may crawl above the water line of your tank as they often become exposed in the wild during low tide. Mine usually end up in the sump where they can get above the water line and come back down when they want to.
Turbo Snails : They are big and clumsy. They'll knock over or move anything that isn't glued/nailed down. Actually I had one knock a coral frag that was glued to a a plug right off the plug. But, they will attack hair algea. Mine are now escargot size and I moved them to the sump because they were always knocking corals off the rock and plugs they were attached to.
Banded Trochus - Trochus spp. (Tiger Trochus Snail, Banded Trochus Snail, Black and White Snail, Spiral-Top Snail ) Algae eating machines. Ideal size, won't knock over corals/rock, and can move very fast. Will also eat hair algae.
Stomatella Snails : Will breed in your tank! You will only need to get 4 or 5. They're great algae eaters and stay very small.