I haven't seen an apple snail in ages, they're federally banned from crossing state lines to my knowledge. Guessing you meant bladder or pond snails? Either of those I see as a net positive in a planted tank, but if you have lots of fish and feed more, they'll put out some numbers for sure. Much like fleas on a dog or cat, it's much easier to avoid an outbreak than to treat one. 3/4 cup of plain household bleach (not splashless, scented, etc.) per gallon of water will kill snails and eggs in about a 2 minute dip. Rinse well in water, then dip in a solution with a strong dechorinator. Once they've established in your tank, it's an uphill climb. General maintenance and manual removal is safest. Assassin snails, dwarf chain loach, or so forth that eat them can help. Competing snails can help too.