Thanks for your answers guys,
First of all, and regarding Randy's question. I was wrong indeed when calling it bleaching. It is STN, tissue necrosis that advances slowly from the base of some corals up to the stems.
I forgot to mention that i have a B.Sc in Chemistry
so i know a lot about water analysis and reactions.
If i need to bet on the cause of the problem i would put it all on low Alk. i think it both causes pH swings and makes ortho phosphate more available in the water (which then feeds the Cyano).
The problem is that i do not have a pH electrode now
... I have the Hanna instrument but not the electrode (you have to replace it every 1-2 years and every such electrode costs here 100 Euros!!!)
About the alkalinity,
I was wondering how fast can it drop? I was shocked yesterday when i found that it's only 5.5 dKh!!! it was about 8.5 dKh about 2 months ago. I am measuring Alk. with Elos test kit which i'm sure that is quiet good. According to your experience, what is the steepest decline in Alk.?
I am working with Kalk reactor, but disconnected it for 2 weeks, because i suspect that the media inside it generates phosphate when it's disolving.
Could a disconnection of the Kalk reactor for 2 weeks affect the Alk. to such extent? According to my experience it definintely shouldn't!
Thanks for any answer...