[Canadian]

There's way too much going on in a reef aquarium to directly associate pH with the solubility of gases in the water as affected by temperature. For one thing, lowering the temperature of the water would likely cause the metabolic rate (the sum of the catabolic and anabolic reactions) of all of the cold blooded animals in the aquarium to decline - for example: aerobic respiration would likely decline, and if you consider that one of the primary components of aerobic respiration as well as photosynthesis is an increase in the H+ gradient which is used to fuel ATP synthesis, and that pH is a measure of the hydronium ion concentration, then it simply makes sense that the lower temp will have an effect on the metabolic reactions of the organisms within the aquarium, and thus have an effect on the pH.

Among other things. And I fully acknowledge that the above is a gross simplification of things (like the fact that cell membranes and their permeability would have significant effect).