In my opinion, the pH range from 7.8 to 8.5 is an acceptable range for reef aquaria, with several caveats. These are:
1.
That the alkalinity is at least 2.5 meq/L, and preferably higher at the lower end of this pH range. I base this statement partly on the fact that many reef aquaria operate quite effectively in the pH 7.8 to 8.0 range, and that most of the best examples of these types of aquaria incorporate calcium carbonate/carbon dioxide reactors which, while tending to lower the pH, keep the carbonate alkalinity fairly high (at or above 3 meq/L.). In this case, any problems associated with calcification at these lower pH values may be offset by the higher alkalinity.
2.
That the calcium level is at least 400 ppm. Calcification becomes more difficult as the pH and calcium levels fall. It is not desirable to push all of the extremes of pH, alkalinity, and calcium at the same time, so if the pH is low and cannot be easily changed (as may be the case in an aquarium with a CaCO3/CO2 reactor), at least make sure that the calcium level is normal to high (~400-450 ppm).
3.
Likewise, one of the problems at higher pH (anywhere above 8.2, but progressively more problematic with each incremental rise) is the abiotic precipitation of calcium carbonate, resulting in a drop in calcium and alkalinity, and the clogging of heaters and pump impellers. If you push the pH to 8.4 or higher (as often happens when using limewater), make sure that both the calcium and alkalinity levels are suitably maintained (that is, neither too low, inhibiting biological calcification, nor too high, causing excessive abiotic precipitation on equipment).
4.
Transient upward spikes are less deleterious than transient downward spikes in pH.
