Settling Sediment

Question:

Why did my pond water change from a murky brown color to a transparent aqua overnight?

Answer:
Sounds like the suspended material, whatever it was, settled to the bottom. Many things can make that happen: The pond is now in balance ecologically; some enzyme you used finally worked. Most murky ponds fix themselves if we just have patience. I have often said that the pond teaches us patience. Glad for you. Whatever you have been doing, keep doing it.