OK, this is easy to explain (I hope
). I may have gone into this in the other thread, but no matter.
In any aquarium, the accumulation of organics and the slow decomposition of these by various bacteria (primarily in the substrate) produces ammonia and CO2. We can forget the ammonia here as it is the CO2 that is directly involved. This produces carbonic acid, which naturally lowers the pH. The extent to which this occurs depends upon the GH and KH (the buffering aspect as you know) and obviously the level of organics accumulating. There are other factors too, all of which is why even with the identical water the pH may be different from tank to tank, as in my own fish room.
Once the biological system becomes established, which is usually over the first several months, and provided you maintain a regular routine of water changes, filter cleanings, suitable fish load, and not overfeeding, the system will stabilize at some level depending upon the factors affecting it. Live plants if present will be part of this too. Over time this biological system will become quite stable. Provided the fish species are suited to the GH/KH/pH of the system, no problems, no matter what the pH may be.
Freshwater fish must adjust the pH of their blood to equal that of the water in which they live. This is why any sudden and significant fluctuations can be deadly. There are normal diurnal/nocturnal fluctuations in pH in nature and in the aquarium; this is over a 24-hour period and is (or should be) relatively minor, and fish has no issues. The pH will be lowest at the end of the night/beginning of daylight, and highest at the end of daylight/beginning of night. This is caused by the accumulation of CO2 during darkness (thus raising pH) and reduction of CO2 during daylight (thus lowering the pH). This is especially noticeable in planted tanks. The fluctuation can be several decimal pints, say from 6.2 at dawn up to 6.5 or 6.6 at dusk, then lowering back to 6.2 during the darkness of night. This is why you need to test pH at roughly the same time each day if you want to ascertain stability or not in general. Obviously testing pH at a different time for several days running might cause one to think it was bouncing all over the place, when in fact it is extremely stable within the 24-hour period, which is what we want.
Now, water changes. The larger in volume, the more stability the biological system will develop. And this is quite strong once established. My tanks that run at or below pH 5.0 (the lowest I can measure) get a 70% water change once each week, using fresh water with a pH of 7.0 or 7.2, but the pH in the tank never rises more than one or two decimal points, and after a few hours is back to where it was. If the source water had a GH and KH above zero, this would play out differently as the tank would also maintain those values for the most part.
To your numbers. If the pH is remaining in the 6.0 to 6.2 to 6.4 range over a period of days, and your tests are carried out at the same time each day you test, you have nothing to worry about. The fish will adjust their blood pH to this same level, and be very happy (again provided they are species designed to live in this slightly acidic water, which cories and gourami certainly are).