I recently completed the ascorbic acid treatment as described in this sites thread. The results were amazing. The ugly brown iron stains are totally remove and the liner is blue once again. I am now bringing the chlorine back up slowly as directed. I don't know if this is a side effect, but I now have trouble with the DE filter rapidly rising pressure in a short period of time(4-6) hours. I figured the DE may have been contaminated with all the chemicals so I lived it and put in new DE. But the problem persists. I cleaned the filter with the Leslie's cleaner before I opened the pool in late May. Any ideas?