What's the best way to make sure Snails do not enter the Aquarium on new plants?
Dip them in a molluscide like potassium permanganate for a period of time (typically 20 minutes, but depends on the concentration).
It has to be 100% effective, hell maybe 200% effective.
There is no such thing as "200% effective". If it works completely, it's 100% effective. If it doesn't work completely, it's some percentage between 0 and 100%. If it doesn't work at all, it's 0% effective.
Snails are quite useful animals, and in tanks with a sandy substrate, Malayan livebearing snails are well worth adding. Nerite snails are hands-down the single most effective algae-eaters in the freshwater hobby, far better than any catfish or shrimp. What comes as a surprise to some people is that snails are unable to break the laws of physics. To hear some people talk, you'd imagine they somehow multiply in numbers without any need for food. Obviously that's rubbish. What snails do is convert uneaten food and organic detritus into snails. The messier (the more badly maintained) your aquarium, the more snails you'll get. Since snails breed inside filters, among other things you'll need to clean your mechanical filtration media on a regular basis. Almost everyone with a snail problem also has a hygiene problem, so snails should be viewed as a symptom, not the problem itself.
Snail numbers are not difficult to control. There are plenty of snail-eating animals, my favourites being
Clea helena, the snail-eating snail. Loaches and puffers will make some headway into them too, and to a lesser extent so will various cichlids and catfish.
Cheers, Neale