Snails

Pest snails come on plants. They are usually one of two types, either tiny flat spirals (a species of ramshorn) or those called bladder, tadpole, trapdoor or pond snails https://en.wikipedia.org/wiki/Physidae I have both of these in my tanks. They are harmless and are actually an important part of a tank's ecosystem. They only become a problem when they overrun a tank, because they look unsightly.

Having a few of them is a good thing but they will multiply rapidly if they have plenty food. This means not overfeeding the fish and removing bits of dead plant. There are ways to reduce their numbers.
Squash them on the glass walls of the tank - the fish will enjoy eating them.
Using a snail trap. This is a jar with a screw top lid. Punch holes in the lid into the jar so that the metal spikes from the holes go inwards. The holes need to be big enough for the snails to get through but small enough that the fish can't. Bait the jar with a piece of lettuce and place it on its side on the bottom of the tank last thing at night. In theory, the snails will crawl into the jar but won't be able to get back out because of the spikey bits around the holes. In the morning, remove the jar and dispose of the snails. You can just put the lettuce on the bottom of the tank (weighted or it will float) but in my experience by the time you get up all the snails will have crawled into the foliage 

But don't use a snail killing chemical as they are not good for the fish.

That's a nifty bit of equipment  I just suck mine up during a water change.

I know this is an old post but I thought I'd comment anyway, when I had a pest snail problem I found the easiest way to get rid of them was by adding more Nerite snails, the nerites out competed the pest snails for food and the pest snails died off