I don't think just one fountain is an instant win for poison decks. But because it's reusable it should cost
By the time you take this out opponent probably poisoned you for lots of points. This just begins a race. Poisoner should keep up when they see this out.
Purify costs just 1 and cleanses everything.
I think, UnUpped:
remove 3 poison counters gain 1 life Upped:
Remove 3 poison counters gain 3 life.
even then, I would rarely use this card.