If you kill all of those snails, your house will smell like a sewer.....You need to introduce something that will eat them, or drain the tank and put it outside for a bit.
I say let the tank dry out for a long time. Get rid of the gravel, and all the decorations let them sit in the hot sun high and dry for a good long time. The tank I would not leave in the sun but let it dry out and let it sit high and dry since you have another tank that your fish are living in now. Someone said use bleach ... that is not a good idea.
Id like to do the "dump outside on a tarp" idea, but we had some major flooding last night and some of these guy might make their way into local bodies of water. It would be like the start of a horror movie