pH 8.4 is just fine. Up to about pH 8.55 is also fine. Above that, and the higher it is the worse it is, precipitation of calcium carbonate can become a problem on heaters and pumps, etc.
These have more:
Reef Aquarium Water Parameters
http://www.reefkeeping.com/issues/2004-05/rhf/index.htm
High pH: Causes and Cures
http://reefkeeping.com/issues/2005-03/rhf/index.htm
from the second one:
What is the Acceptable pH Range for Reef Aquaria?
The acceptable pH range for reef aquaria is an opinion rather than a clearly defined fact, and will certainly vary based on who is providing the opinion. This range may also be quite different from the "optimal" range. Justifying what is optimal, however, is much more problematic than justifying that which is simply acceptable. As a goal, I'd suggest that the pH of natural seawater, about 8.2, is appropriate, but reef aquaria can clearly operate in a wider range of pH values with varying degrees of success. In my opinion, the pH range from 7.8 to 8.5 is an acceptable range for reef aquaria, with several caveats. These are:
That the alkalinity is at least 2.5 meq/L, and preferably higher at the lower end of this pH range. This statement is based partly on the fact that many reef aquaria operate acceptably in the pH 7.8 to 8.0 range, but that most of the best examples of these types of aquaria incorporate calcium carbonate/carbon dioxide reactors that, while tending to lower the pH, also tend to 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. Low pH stresses calcifying organisms primarily by making it harder for them to obtain sufficient carbonate to deposit skeletons. Raising the alkalinity mitigates this difficulty by supplying extra bicarbonate.
That the calcium level is at least 400 ppm. Calcification becomes more difficult as the pH falls, and it also becomes more difficult as the calcium level falls. It would not be desirable to push all of the extremes of pH, alkalinity, and calcium at the same time. So, if the pH is on the low side and cannot be easily changed (such as in an aquarium with a CaCO3/CO2 reactor), at least make sure that the calcium level is acceptable (~400-450 ppm). Likewise, one of the problems at higher pH (above say, 8.2, but getting progressively more problematic with each incremental rise) is the abiotic precipitation of calcium carbonate, resulting in a drop in calcium and alkalinity, and the resultant clogging of heaters and pump impellers. If the aquarium's pH is 8.4 or higher (as often happens in an aquarium using limewater), then it is especially important 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).