The three tests George notes are critical(Mg,alk,calcium).Water changes can help but ain't enough as corals grow. Since I have been more consistent in checking these weekly my corals have great color, cyano issue has dropped. This is the best thing I have done since setting up my 55 gal tank.
I currently dose with BRS 2 part and their magnesium. I am still titrating my daily dose but putting 50 ml of each daily seems to be holding it steady. At first I thought I was dosing way too much but have come to realize all the sps,coraline,other calcareous life forms in my tank are chewing it all up. BRS has a great calculation on their site to dose for any of these. Paul is probably going to point you to his cheaper homemade alk,calcium,magnesium. I started with BRS cause I like the dosing jugs and it makes mixing pretty basic. Next time I will go the route of Paul and make my own.