Hi there,
Sounds all too familiar to me.
I have first hand experience with these snails as well and they are so hard to get rid off once they have established themeselves.
I have tried snail baiting them as suggested by the previous post but this is a long proces but will never eliminate all of them.
You can try and use a product available in aquariums called snail rid. However this is only transient and will only kill surface snails. But I am sure your snails are in the gravel as well, so this may not be effective either. At least that is what I have found out anyway. I tried doing all of these about 6months ago and thought I had them beat, but 6months in and well their back!!
Only 3 weeks ago, I tried a different approach. Drastic, but I didn't want to chance it again.
What I did was the following:
Quarantined discus, tetras, BNs, Peps, royal whips, Clown & Butterfly plecs
Live plans in the bath for a week, treated with snail rid and throughly washed every 2nd day.
As for the tank & gravel I completely emptied the tank. The gravel, spread on a cloth matt & baked in the hot sun for over a week. This killed all the snail & more importantly the eggs. If this is too mich trouble, then just buy new gravel & start a fresh. If you want to keep the gravel which you dried, you must run the filter with heaps of carbon to remove the nitrites from the decaying snails. Oh, by the way don't forget to clean your filter either as those dam snails will live in your filter.
Bottom line, make sure your cover all bases. Once all this is down, re-setup your tank again. It is heaps of work, but it will be well worth it.
A lesson for me from now on, is never trust the water where you acquire your fish from. No matter how clean & trust worthy the place is, it may always carry snail egss, especially if when getting live plants.
As I said, I diid this a couple of weeks ago and only time will tell how successful I am. But as this is round two, I am confident will will work.
Summary; quarantine your fish. Wash everything & use chemicals for plants, kill snails burried in the gravel by drying them out. Then re-set up your tank.
I hope this helps, not sure if there is an easier way, but this I am sure will work.
JJ