Lilras, you mention "hard" water but refer to pH numbers, so I just want to ensure that there is no confusion, as the two are very different things, though related...somewhat.
I'll start with pH as that seems to be what you are trying to lower. The pH is what it is due to a couple of other factors, namely the hardness of the water, the carbon dioxide (CO2) and the organics in the substrate (mainly). The source water, tap presumably, will have a level of hardness that will, depending upon what the bicarbonate hardness is, act as a pH buffer to resist changes. Attempts to lower pH with chemical preparations will almost always be unsuccessful because the hardness buffering resists this, and fluctuating pH can result which is even worse than a steady pH that may be higher than the preferred range for the fish.
At some point the buffers may be exhausted, and this is where CO2 and organics play a more major role. The breakdown of organics by bacteria in the substrate releases CO2 and this forms carbonic acid which causes the pH to drop. The amount this occurs depends upon the initial hardness, plus the organics--and these occur from the fish load, plants, fish food, etc.
Without addressing the hardness, you will be unable to have much effect on the pH. The softer the water, generally speaking, the more the pH will tend to lower.
Yu can read more in my article on this topic, here: http://www.tropicalfishkeeping.com/f...quarium-73276/