Honestly, in my experience most "cleanup crew" animals are overrated. Hermit crabs just climb all over corals and kill snails for their shells; many crabs (especially Sally Lightfoots) become aggressive opportunistic predators as they get bigger; and most stores suggest overstocking with snails, which leads to their starvation.
That said, I think some snails are helpful when they are lightly stocked (so the poor things don't starve to death). I found that when I ignored the however-many-per-10-gallons "rule" that I was given by stores, and just added a couple of animals, they suddenly quit dying mysteriously (also, I don't have any wretched hermit crabs to attack them), because they didn't have so much competition for food. You have a ton of algae-eaters in that tank. If it were my tank, I would get rid of all of them except for either the astrea snails or one turbo, and then supplement their diet with nori. If you have a lot of problems with algae, try to address the source (possibly too many nutrients in the water due to overcrowding, overfeeding, and/or an inefficient skimmer), which ultimately is more efficient than trying to control it with algae-eaters. Good luck!
