You can use a diluted solution of potassium permanganate to sterilize plants of  diseases and snails. Use a solution of half tablespoon in one gallon of water and soak for about 15 minutes. Thoroughly rinse the plants with clean water afterward to remove any traces of chemical.

As far as getting rid of any remaining snail and eggs in the tank, you can use a weak diluted solution of chlorine bleach. Use the regular bleach instead, not one mixed with detergents since detergents can leave residue that are difficult to remove.  Thoroughly rinse the tank afterward and let it air dry.  Once you refill the tank again, use an extra amount of conditioner to make sure all the chlorine bleach is neutralized.