Hold off on that trip to Walmart. I think the high FC levels may be bleaching out the DPD test....rendering it useless.
I suggest you add no more bleach until you can test with the TF-100 or take a sample to a pool store and let them know it may have high levels of chlorine.
I suggest you add no more bleach until you can test with the TF-100 or take a sample to a pool store and let them know it may have high levels of chlorine.