Attempts to lower pH frequently fail, or are temporary at best. And fluctuating pH is more stressful and debilitating to fish than a steady pH that is not too far afield. And, one more detriment...all these chemicals going into the tank do affect the biology in various ways, and add TDS (total dissolved solids) which can be very harmful to some fish.
Before attempting any adjustment to pH, you must know the GH and KH of the water. This will be the source water, out of the tap (or well), and this is unlikely to change much in the aquarium unless something is being added that specifically targets it. And obviously this is another source of pH-affecting substances.
The KH (carbonate hardness) acts as a sort of buffer, to maintain a stable pH. The degree to which this works depends upon the KH level, and any attempt to lower the pH by any method (including peat, leaves, wood) other than diluting the water with "pure" water will be countering the buffering capacity of the KH.
So, if you can give us the GH (general hardness) and KH of your source water [this you should be able to ascertain from the municipal water folks, perhaps on their website, unless you are on a private well], it will be helpful.
Also, what is the reason you want 6.5? I assume for some particular fish. But 6.5 to 6.8 is miniscule to fish, and even the 7.2 may be perfectly acceptable. I appreciate the significance of above and below 7.0, so this may need some intervention depending upon the fish. But assuming the fish in question is soft water, hence wanting an acidic pH, the pH adjusting chemicals like Waterlife is going to actually be more harm than benefit. I can't say much more without knowing the specifics, being the fish species and GH/KH. Also, are live plants in the tank? And are there any calcareous substances? Both of these also play into the equation.
Byron.