While my tank was cycling (fish in) I noticed my ph would drop so I tested my tap waters KH & GH and if memory serves I had a reading of 53.7 ppm GH/KH so I began adding crushed coral to my canister filter (about 1 cup) and my ph held between 7.2 - 7.6 (ph out of the tap is 7.6). So between adding the crushed coral and doing daily 50% water changes my ph stayed stable.

Now that my tank has cycled I started testing my water and doing water changes only on a weekly basis and my ph is again showing issues of instability. During the day it tests at about 7.23 but has dropped all the way to 6.62 by the morning then it starts to climb again. Adding fresh crushed coral to the filter and adding an additional cup of it (hanging in a media bag in the tank) hasn't really helped the problem. I've also done 3 days of 50% + water changes and that hasn't made much of a difference either. Water parameters are 0 ammonia / 0 nitrites / 0 nitrates (usually 5-10 but have just done 3 days of pwc's).

I tested the tap water again this evening - PH 7.6 out of the tap and the KH/GH were as follows: KH = 3 drops to turn water yellow (light yellow not dark). GH = 6 drops to turn water green (light green not dark).
What exactly is my ppm GH/KH?

Even more confusing is my 10 gallon QT tank PH has tested 7.6 for three days. No live plants in there or driftwood though, I do have 5 live plants and a 10" x 2" piece of driftwood in the tank with the ph issues.

Any advice would be greatly appreciated.