Hi Guy's,thanks for the support's,it means a lot for me that i am not alone.
Whe tried to think what causes this whit some other expierience guy's in the hobby and some professional people but too be honest,we can't find it so it is very frustrating.
By now all te acropora's are out off the tank,not a single one left;still all the other corals and fish are ok.
Some ricodea's are schrimping but mayby it is because the wter is not clean anymore from the dying acropora's.
This is the way they died so mayby somebody recognize this,at first the polips are away and a few hours later the corals glow,they almost give light in the dark,than very fast they become dust grey and a few hours later you can blow off the dust,so normal bleaching.
In the weekend all the cellar tanks will be total cleaned and all will be filled whit natural seawater ,around 2500 liter that i will collect buy the wholesaler so it is already heated,it will loose a few degrees but it will be better as fresh water from only 5 degrees C.
As soon it is correct in temperature i will slowly mix this whit the maintank and than we will wait what the coming weeks will bring.