jbizzle is correct in suspecting iron in the water. If it is indeed metals, I would proceed as he suggests as a quick fix.
However, keep in mind that the ascorbic and citric acid based stain removers will dead your chlorine level. You're in good hands with the SWG, maybe keep it low and increase slowly until you're back to a good Cl residual. Also, be sure to follow up with a sequesterant and backwash your (advantageous) DE filter once the problem clears. More frequent backwashing may also behoove you, as you are backwashing the sequestered metals out of the equation. Depending on how much iron is in the water, you should get some nice autumn colors in the sight glass. If the fill water is from a well, you may want to look into the available solutions for removing the metals from your fill water, be it a hose attachment to a full fledged well filter system.
There's actually a few more steps you can take, but first confirm that it is metals causing the staining.
Ahh, the old cycle of iron staining.