So Fizz's W has no cap against monsters?

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Why should it? Most Fizz junglers go AD so what would it matter to deal a bonus 20 damage per second?

Edit: It does deal more than 20 damage eventually, but not at rank 1 which it should be until at least level 9, unless you choose to put a point in it at level 4,5 or 8.

What does it matter if they go AD? It is + 4 / 5 / 6 / 7 / 8% of target's missing health magic damage per hit from the active that this thread is probably about.