If the brown stain won't brush away...

[docsobeck]

i agree that it must be an algae issue, but i swear there is no roughness, pits, divots, or anything at all different about the places the stain appears. very strange.

it's a light brown color, not yellowish at all, but not dark and really noticeable. it's easier to see when the sun is bright; when it goes behind a cloud, it's very difficult to see. guess i'll just have to be more vigilant about keeping my FC level up.