An acid in the water will lower pH, but you'd be better served to discover the causes. What pH is it heading towards? Bear in mind that 8.6-8.7 isn't unhealthy (I think Calfo did a series of rants on that topic) so don't assume you *NEED* to be at precisely 8.3 at all times.
What's in the water? Just aragonite? Crushed coral? Silica? Live rock? Any plastics beyond powerheads and the like? Any rock that isn't coral-based? Additives? Kalk drips? Buffers?
This question is so much easier for freshwater... You just tell them that they need to change their gravel.
