Marco I placed a drum with static Kaldnes in the vortex itself. My second chamber used to have brushes as well but now holds fluid Kaldnes (kaldnes with air).
In case you missed it some picture of my conversion (bottom of page one)
http://www.koiquest.co.uk/forum/viewtopic.php?t=748
It's all a matter of of being able to adapt the vortex to take the drum. I did so by drilling a hole in my vortex and fitting a knife valve with a rubber ring. All you do is place the rubber in the hole and insert the valve with some lubricant. The hole is then water tight.
The water passing through is now quite clean. I do blow air through my J-mat to keep any detritus from setting there.
This would be a much better option than trying to put kaldnes in the second chamber if you ask me.
B.Scott