Tank crashing...could it be due to these?

[Diomedes]

Interesting problem...but I don't see how phosphates that aren't being detected are contributing to the RTN problem.
My reasoning...
Whether it is DOP(Dissolved Organic PO4), DIP(inorganic) , POP(particulate organic), PIP (particulate inorganic)... These types of phosphates are linked through a cycle in your aquarium. If you are reading a zero when measuring DIP (most test kits - Salifert incl.) then
it is unlikely that your other phosphate based constituents are out of control.
There is a neat article with a brief display of these cycling PO4 forms:
http://www.reefkeeping.com/issues/2003-04/eb/

This doxycycline business is kinda freaky...I wonder how Julian came to these conclusions? (doxycycline/nitrofuazone and elegance corals) and how they apply to RTN in SPS corals. I would love to see his research but there doesn't seem to be any...I would appreciate a link because I couldn't find anything more than his speculation about the rising mortalities among elegance corals.

Brad, Mason,
What about the growth of your acros and other SPS and their changing water flow requirements as they become larger? I know you have been doing this a long time, but are these colonies getting larger or are you fragging them a lot?

What are your pH fluctuations between day and night? Have you considered turning your reactors off(down?) at night or dripping Kalk only at night to compensate for low nighttime pH?

Hope these thoughts help...

Stephen