I recently recharged a bag of purigen. Since I swap them out, I left the other to dry. I noticed that after the bleach soaking, it was bright white again. After drying, it's turned brown again. Any ideas as to why this would happen? Should I recharge it getting it to the pure white color before placing back in the tank? It was white when I bought it, I'm thinking it should be white when I put it back in.