1. Don't know answer, if it turns dark brown , just recharge or replace. Never left one in the tank once it was discolored...
2. is your tank overcrowded or underfiltered? I love purigen! I use it on my display tanks when I go on vacation as a safety measure and then instead of an RO unit I filter my water through purigen in my storage tank.
I use this water for my display tank water changes....