Actually there are alage eaters that can live in brakish water. The Algae Eating Olive Nerite Snail. These little fellas are actually about the size of a dime or a marble, and have neat little barnacles growing on the shells. Probably about the best algae eating snail for aquariums in the world, as they DO NOT reproduce in freshwater, but only in brackish or saltwater. They may lay "pretzel- salt-looking" eggs, but they won't hatch unless the eggs are laid in, at least, brackish water.
STOCK about 1 per gallon or more. a brackish water snail (brackish water is where they are prolific. Excellent algae eating snail for freshwater aquariums. Will NOT harm plant life.