Some places use a common goldfish for snail patrol, but I doubt they eat them all. That said, most cichlids like snails from what I understand. I don't know how cold 20ºC actually is in Fahrenheit, but if it's at least 72ºF you could use a convict for a short term extermination project.
Another method for greatly reducing the population is to put meat in a jar and sink it. Then, around three in the morning when they are all over it, take the jar out and kill 'em all. Repeat. After two or three days you'll only be seeing one or two.
Remove your gravel if you can and wash it in vinegar. Rinse well. Let it dry somewhere. Rinse well again and return it to the tank when the snail problem is resolved.
The old school method was pennies. I think three pennies per gallon for five days or something similar. The hard part is that you can't control the copper dosage very well because of the huge variety of factors involved in decomposing the pennies. And then you have to remove all the copper from the water. But the snails will all die. Check your plants for copper vulnerability if you're going to try it with pennies.