There's a thread about how recharging Purigen can be potentially toxic if you use CSM+B. You've probably seen it but I'd hate for something happen to those discus.
The cause of the problem was clarified. It was the use of certain brands of water conditioners that contained amine-based slime coatings during the regeneration process that caused the Purigen fouling. CSM+B is fine with Purigen.
DC is light green. Solution hasn't been changed in awhile but reacts normally when brought into the air, etc. The needle valve is marked just before the point where shrimp start swimming around erratically, a symptom I consider a precursor of CO2 toxicity.
Several days ago I switched to a spray bar, so maybe I'm losing more CO2 to the atmosphere now. I think I'll whip up some more solution and see if the level can be boosted to compensate.
Your CO2 seems fine then since you're below the threshold of your shrimps maximum tolerance. Your flow seems good as well, according to your posts in the other thread. So you can rule out the CO2 and the flow problems. You said the lower leaves darken and fall off. Maybe it's phosphate?
I ran into algae issues first and decided to up the CO2 beyond the shrimp-safe level, since there weren't any left at that point. I'm planning to add more fish so the P level is going to jump anyway. In the meantime I'm boosting the KH2PO4 dose.
I've also cranked the flow from the spray bar so there are ripples and lots of flow, meaning I needed to add even more CO2. The fish are happy with the extra O2 but now I have a waterfall of floating plants.
Back to your original question, Rainer - I've been using Purigen 24/7 in my planted discus tank for well over 2 years now, and wouldn't be without it for producing & maintaining crystal clear water.
I get Purigen in the 250 ml jar and bag it myself (double-bagging to ensure no particles escaping), and have re-charged each supply at least a dozen times (about every 3 weeks or so) before it begins to lose any effectiveness, and I need to replace it.
I run 4 bags at a time, one in each of my 2 filters, and the other 2 soaking in conditioned fresh water, after a re-charge, awaiting to replace the other 2 as they get dirty.
Here's my tank if you care to have a look: http://s1105.photobucket.com/albums/...ul/lolliblues2