Yellow water

Oldish water with MH over it will turn yellowish. I believe the UV? or bright light acts on proteins in the water.
 
I agree with the carbon usage, since that will be the easiest way. That is one reason why I use lots of it continuously. The compounds are natural tannins from algal matter and the like--UV would actually help break them down, not create them, which is one reason why UV sterilizers can keep water so clear (the same goes for ozone).
 
Another vote for carbon. Should clear it up in a day or 2.
 
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