I continue to see questions like this on yahoo answers about changing the ph of your aquarium.  New fish keepers will attempt to alter the chemistry of their water with whatever potion they find on the fish aisle of their pet store.  The reality is that almost anything you do to change the ph will be worse for the fish than the ph to begin with.
If the ph is changing - ie, your tap water is 7 and your aquarium is 5.5, then you likely have depleted the kh (carbonate hardness) if the water.  The best way to get your tank's ph back to the tap water ph is through water changes.
There are two ways that I will say are "ok" for adjusting ph.  To lower ph, add dissolved CO2 into the tank.  The CO2 will form carbonic acid which will lower the ph nore naturally.  To raise the ph, use a product called "kh buffer".  Some tap or well water is naturally deficient in kh.  Having a low kh will make your water prone to ph swings to a much lower level.
Toss the "ph 7", "ph up" and "ph down".  They are bad news.