I don't think just one fountain is an instant win for poison decks. But because it's reusable it should cost
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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:
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remove 3 poison counters gain 1 life Upped:
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Remove 3 poison counters gain 3 life.
even then, I would rarely use this card.