I'm going through the same problems right now the difference is that I have MH + T5 lighting so I know for sure it's not a lighting issue in your tank. At first I thought it could be the amount of light/ photo period on my tank which was 10 hours after making adjustments I thought it was possibly the distance of the lights to the water but bottom line lighting does not appear to be the issue. I was also running the WM bio pellets with an Eheim 1250 pump in a NextReef reactor and so after reading other peoples posts regarding their negative experiences with their bio pellets I took mine off line, unfortunately it has not made any positive or noticeable difference. I then spoke to one of the guys at vividaquariums and he suggested using GFO (rowaphos) which I had not been using because my po4 has always been undetectable using different test kits, but in my desperation to find a remedy for my dying corals I set up a BRS reactor with an MJ1200 pump running 250ml of rowaphos, again no noticeable difference. Of course people were suggesting that maybe my test results were not accurate so I took some water over to Age of Aquariums (a reputable LFS) and their results almost 100% matched my results, so we began talking about pollutants in the water and the guy commented that although my TDS meter reads 0 that if I have not changed the filters in my RODI unit I could still be getting chloramines and or metals in my water. So to be completely honest I had not replaced any of the filters in my RO/DI unit since I got it 2 years ago, I had only replaced the DI resin. I have now replaced all the filters (except the membrane), and did a 20% water change, added a poly filter and I'm hoping that this will resolve my problem. I do see one of my tri-color valida showing some signs of life but just about everything else is still the same, I'm going to continue doing water changes every couple of weeks as I don't want to cause major changes to water parameters and hopefully getting back to basics will help.
Tank: 120 glass
Lighting: 2 x 250 MH
Sump: Approx 40 G
Bulb: 1 20K Ushio / 1 20K XM
Lighting: 4 ATI 48" T5's on ICE CAP ballast
Light schedule: (currently) first MH comes on at 3:00PM and the second one on at 3:30 PM and off at 9:00PM and 9:30PM
Skimmer: Vertex IN100
Return Pump: Laguna 2400 at about 50% power.
Flow: MP40 X 2
Reactor: BRS reactor with MJ1200 running Rowaphos
Carbon: Chemipure Elite
Dosing Pump: BM T-01
Dosing: BRS 2 part
PH: 8.1
Cal: 450
Alk: 10 DKH
Mag: 1360 or 1380
Phos: 0
Water top off: JBJ ATO
water: Only use RODI provide via air/water/ice RODI unit.
BTW my ALK has always been 9 - 10 and almost everyone I've talked to does not think that 9 - 10 is too high.