By Frank Cowherd (extracted from a PVAS forum thread)
One way to treat plants that have insects, hydra or snails that you do not want is to use alum. Alum is a pickling agent (actually used for pickle crispness, and also used in deodorants and cosmetics) and is sold in some grocery stores.
To remove unwanted organisms from aquatic plants, prepare a gallon of water with about a tablespoon full of alum in it. Transfer the plants to be treated into this water and let stand for a couple of hours. Rinse and put the plants into the aquarium. Higher dosages of alum can be used if there are lots of snails, snails are the hardest organisms to get rid of. But plants will not survive more than a couple of hours in really high dosages. I have kept plants overnight in water containing 1 tablespoon of alum per gallon without causing harm to the plants.
This is a great way to get rid of unwanted organisms from plants you might collect from a pond or stream. Alum is basically aluminum sulfate. It can also be used to remove phosphate, a fertilizer for plants and algae, from ponds, although I have not heard of its being used in aquariums. It removes phosphate because aluminum phosphate is insoluble and thus precipitates, making the phosphate unavailable to the plants or algae.