Adding water due to evaporation is not a water change. It is diluting the bad stuff, but not removing them, so they are still building up.
Natural light isn't bad. I'd be worried about temp. fluctuations. I've my 5. gal. in a place that gets full sun in the winter for several hours a day (cause the leaves are off the trees) and so I have to put up something to block the sun so the tank doesn't go over 80 degrees, which it would if I left it unblocked. Do you have a decent thermometer (one that goes inside the tank, not one that glues to the outside) to monitor the temp?
There are other ways to bring the ph down naturally, such as driftwood. I've not much experience with that as my problem runs the other way and I have to add crushed coral to my tank to bring my ph up.
Do you have a test kit to monitor your water parameters? I know you mentioned ph, but are you also testing for ammonia, nitrites, and nitrates? If not, it's been recommended on this forum to get the API master liquid test kit and do regular tests. When you ask for help with problems, the first questions asked are "are you doing regular water changes and what are your water parameters".
Welcome to the forum.