[emc7]

I have those "i am a horrible keeper" feelings myself sometimes. I know I should be spending less time on the boards and more in the fish room. Loh is right in that you worrying doesn't do a thing for the fish. Do the best you can and let it be.

From exp. I have learned, that when you have soft water, pH can be a quick indicator that you are overdue for a water change. As nitrates go up, pH goes down. It might be better to test for nitrate, but pH is quicker (esp. with a pH pen). Down around 4 you can see the fins start to suffer (don't ask how I know this). And the wider the gap better the tap and tank, the more risky larger water changes are and the more that tank needs one. So, say your tap water is 7, and if you leave it in a bucket for a week it goes to say 6.5, you really do need to change water at 5.5 or soon your tank will be down to 4.
Of course, if you keep to a schedule, you shouldn't get overdue. But fish grow and gradually need more water changed than they did before.

Nitrates aren't the only thing that can lower pH, CO2 can also. But Goldfish are rightly considered "high bio-load" . They make a lot of waste that ends up as nitrate, so it seemed like a reasonable guess that "organic acids" (aka nitrate) were lowering your pH.