If you sit in a chair and breathe as rapidly as possible (hyperventilate), your blood pH will increase...ie., your blood will become more alkaline. This is because you are blowing off more CO2. In solution (blood or aquarium water), CO2 + H2O will form carbonic acid. Increasing the circulation of your aquarium water could, conceivably, mimic rapid breathing, causing more CO2 to be released and hence, the pH should increase somewhat. Although, this may only be a negligible change. This increase in pH has essentially nothing to do with water hardness, however.