If you have an over population of snails it may be that there is not enough algae for them so then they attack the plants.
If that is the case the the way I thin down the herd is I thread a piece of cucumber with a needle and thread leaving a long string then before the lights go out at night I toss the cucumber piece leaving the string out and then in the morning the cucumber piece is covered with snails and you simply pull the snail covered cucumber piece out with the thread and toss 'em out.
Easy and no harm to fish or plants.
Hope that helps,
vanessa