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