Tank 42 x 31 x 21 high (inches),What are the dimensions of the tank? What kind of metal halides, what wattage and brand? Is the T-5 a 4bulb t5ho, if so what brand. Exactly which Flourish supplements are you using? What are your nitrate and phosphate readings?
Maybe you could put in a link to the fixture. This is all one fixture? with 2 x 150 watt metal halides and 4 x (?watt) T5 (HO or NO)
I use a water hardness monitor by american marine to monitor and maintain stability in the water hardness the hardness is measured in seimen units and stays in the upper 3 hundred's when needed I add sea chem equilibrium .Are you adding anything to the ro/di water for your water changes?