Ph level in fresh water tank

Question
I have an established  45 gallon fresh water fish tank that I am having problems maintaining the PH level at 6.5 to 7.0. I have silica gravel on the bottom of the tank and that is new to the tank and when my Ph level started to drop. I have cleaned the tank and changed half the water with Ph level 9.0 tap water. I did a water change 3 weeks later, but still the Ph level remains low. I also used Ph up that did not maintain the Ph level. Please I  am loosing fish and my patients with the tank.  

Answer
Hi Tina,
Hang in there! First of all I don't know what kind of fish you have but I have found that usually even though the ph might not be ideal, most fish will adjust to a reasonable ph. Many times when we try to regulate the ph to be the exact level for a particular fish it does more harm than good,such as resulting in more fish death than would occur if the ph wasn't being adjusted.  One way that seems to work for most people is just to do very frequent water changes, such as at least once or even twice a week.  Then the ph will stay at a somewhat level range instead of the ups and downs from using chemicals to control it.  Also, if you don't do frequent water changes the ph will always drop lower and then when you do a water change it will always go way up. If there is a  long time between  water changes the ph drops and the fish adjust to that level and then when a water change is done the ph goes up from the addition of the replacement water and it is a shock to them and can make them very ill. The same is happening whenever you are trying to adjust your ph with the chemicals. Try doing the frequent water changes and see if your fish adjust to that better than you trying to regulate it. Try removing about 1/3 of your water two times per week and maybe that will maintain a ph that your fish can adjust to. Whenever you use any ph chemicals they usually cause alot of fluctuations. The only chemicals that claim to not be as hard on fish are these :http://www.drsfostersmith.com/Product/Prod_Display.cfm?pcatid=4124  I have never used these and if you think you still want to try to adjust the ph you might want to try these. Usually, unless you are keeping discus or some other cichlid that requires exact water conditions and are more delicate, you could probably just do the water changes and they would adjust.
Best of luck!
Karen