Is My Waterfall Killing My Frogs?

[Diver]

They need much better ventilation, a full screen top at least. Moist, stagnant air will indeed kill them. 60% humidity is great, (I don't know who would suggest 90%, that's complete nonsense unless you're trying to bred them in a rain chamber) but in reality none of the cheap hygrometers give you more than a vague clue anyway.
You don't want ANY condensation running down the glass.