Do not get fish for snail control. A lot of people go for Clown Loaches ... not realizing you need a 125g+ tank for them.
Snails are great for a tank, they do lots of positive things. They will only reproduce to the level of nutrients that will support their population. If you have millions, you are too much food for them.
You have an algae problem though from that picture. That happens from over feeding, and having your tank light on too long during the day. It can also happen from other factors, but live plants really help in cutting back nutrient levels (they use it before the algae can).
But, if you really want them all gone ... that's hard to do. One method is to put a piece of vegetable in at night, then in the morning before the lights turn on you remove it and toss it. It should be covered in snails. Basically it works like a trap. At the least, this will help control their numbers but I really suggest fixing the problem that's causing them to reproduce in such numbers in the first place.