I agree, snails can be a good thing in your tank and part of the little ecosystem, however, they can get out of control and be a mess...lol
They most likely came in on the plants
Usually causes of over population is from overfeeding and left over food in the tank and not enough water changes with vacuuming
You don't want a massive die off of snail in the tank by using chemicals or trying to starve them out due to causing water quality issues from ammonia spikes from the decaying bodies
The best way I like to use in control- is to trap them by using a small clean either glass or plastic container with bait, algae wafer work well or a piece of lettuce or any green veggie will work
I place the small container with the bait in the tank at light out and remove in the AM with light on..usually full of snail, and repeat as needed
Proper disposal of the snail is required-either by placing them between newpaper and smashing them and throwing them in the trash or you can smash remove the meat and feed to the fish in small amounts
Never release or flush....due to environmental issues....