You said you re-charged Purigen?. That is like poisoning your bb with bleach.
If it's not a bacterial bloom from destroying your beneficial bacteria, then the cause is something unheard of.
I throw Seachem Purigen out once it is depleted and turns to coffee ground dark brown. That's why I run two at a time at different intervals. All those stories about re-charging it are pure BS.
EDITS: I thought you were running a Fluval C4........ I must be mistaken ..... you need to filter out the micron particles. EDIT again: Your tap water must be as bad as mine. Please run bottled spring water for a while when you do water changes. Sorry that it costs $1 a gallon.