Answer:
As a 13 year pond addict who lives around oak trees, I found that the tannins in the leaves that fall in/on the water will discolor the water to brown, but you can still see through the stain. If you remove the leaves promptly, there will be less stain; if not, you get brown water and decomposing leaves turning to sludge and nastiness. Most of the chemical clarifiers will not remove the stain. I have to resort a water change, and virtually re-start my pond after all the leaves have fallen. My pond is only about 700 gallons, so this is in the feasible range for my small pond. Hope this helps.
Dianna