Hi Randy I ment suppose I have 5 ppm nitrate in my tank should I try to maintain 4,5 meq alkalinity for maximum coral growt. Or did I misunderstood the article?
In most cases where nitrate levels have been examined in relation to the growth of calcerous corals, the effects have been reasonably small, but significant. Elevated nitrate has been shown to reduce the growth of Porites compressa (at less than 0.3-0.6 ppm nitrate),16,17 but the effect is eliminated if the alkalinity is elevated as well (to 4.5 meq/L). One explanation is that the elevated nitrate drives the growth of the zooxanthellae to such an extent that it actually competes with the host for inorganic carbon (used in photosynthesis and skeletal deposition). When the alkalinity is elevated, this competition no longer deprives the host of needed carbon.17
I think it is a good idea to keep alkalinity on the high side if you have elevated nitrate, but I wouldn't worry about the difference between 4 and 4.5 meq/L. Just at the high end of the normal reef keeping range is a reasonable goal.