to elaborate on Watchers replies, most fish will easily acclimate to those PH levels. It is usually much harder on fish to to fluctuate ph (alkalinity or hardness is really the dangerous fluctuation) or run the risk of doing so. If you don't see any issues, you don't have a need to fix anything.
Tap water is generally laden with co2, it equalizes with the air and then the PH will rise a bit. Set the tap water in a shallow bowl overnight and then test it. It will most likely match your tank PH by morning.
Now if you really need to lower the PH for some reason, there are a few ways that are effective and controllable. the ones that come from the LFS in a can will cause you much gnashing of teeth and possibly loss of life(in the tank I mean).
Peat and driftwood will release tannins (tanic acid) into the tank, these will lower/deplete Kh and therefore lower Ph. a little difficult to regulate, and will require excellent habits and scheduled maintenance to keep steady levels. Driftwood will lose it's punch pretty quickly, but peat in the filter can be added/used at whatever level you desire. Not my favorite but effective nevertheless.
Mix tap water with RO water to dillute the hardness. This is an easy and fairly decent way to lower Ph. you will have to search for a ratio that gives you a good level of hardness. remember that RO has essentially no hardness, so you are using it to dillute your tapwater to a desired level, don't add Ro right type products as these increase hardness in the water you are trying to dillute.
If you have a plant tank the easy logical and highly beneficial method is to inject Co2 in the water. Co2 creates carbonic acid and thus lowers the Ph a bit.
As said there is really probably no reason at all to mess with it and your fish will be better off in most cases if you don't. I played the hardwater game with very little real knowledge for a lot of years, and when I say the stuff in the cans from the fish store will cause gnashing of teeth, I really mean it. Not a game I will ever play again.
dave
