There are multiple possibilities.
Put a chlorine tab on one step; put a couple of vitamin C tab on higher step. Avoid disturbing the dirt when placing the tablets. Remove the chlorine tab after a couple of hours.
If the chlorine tab clears the brown, it's algae + bacteria. If the vitamin C clears the brown, it's iron. If the vitamin C has little affect, and the chlorine has no effect, or only clears the brown partially, it may be dirt or pollen.
Meanwhile . . . order a K2006 from Amazon (see link in my signature), and start testing your water. Report results.
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