I was in a similar situation a few years ago with very stagnant growth and random coral deaths. Besides my nutrients being high, I also had low mag and PAR levels. I think correcting your lighting levels, increasing your mag (especially if the ICP test is accurate), and stabilizing alk/calc as a result of the mag increase, will get you on the right track. I run a 48" Photon V2+ 10" above a 22" deep tank and it ramps up to 75% during the day. As you suspect, you are definitely not providing sufficient lighting at only 10% intensity and at a much higher mounting height.I've still got plenty of intensity to raise them (they are only at 10% currently) and I like them mounted higher to reduce shadowing and any colour separation. I think this is also the height BRS recommended. Sorry, not trying to sound argumentative, just adding information