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.