These products do not work relibaly for most situations. The problem is that you're adding a chemical which can not remove the components that cause a high pH--they temporarily burn through some buffer, the pH dips, then spikes again. So, here's my advice:
First--why are you wanting to reduce the pH? Most fish can be acclimated to 7.6 easily and thrive. Outside of breeding, this shouldn't impact the fish at all.
Second--water chemistry. Must know KH, GH, and water change schedule. Changing chemistry is best done outside the tank, so the fish don't go through the transitiions (changing pH can be stressful, while a stable pH that's a bit high or low is not).