Originally Posted by pokmijn
nitrate - off the chart which i dont know why...
I don't think I'm telling you anything you don't already know, but you're going to need to find out why. Just replacing the water will not make the problem go away - whatever made the ultrahigh nitrates will still be making it, and after a few weeks you'll be back to where you are now. (Not to mention a 100% water change will stress the living heck out of your fish and any corals.)
First off... get rid of your crushed coral. That is notorious for catching debris and elevating nitrates. Get rid of a section at a time so you're not disturbing the sand bed too much, all at once.
You say your source water is great, but where are you getting it from? Tap? LFS
? Source water should be the first thing you look at when you're having water quality issues.