The KH is fairly low, and that is a buffering for pH, so being low it means the natural biological actions in the tank will cause the pH to fall as the water acidifies. This is normal, not a problem as such, unless you have livebearers or other hard water fish. Your fish mentioned earlier are soft water, so this is fine.
However, I would get on a regime of weekly partial water changes of 1/3 the tank. But first, begin slowly; change 1/4 of the tank daily for the next few days. Test pH just prior to the water change to see what it does. It should slowly rise. Test the ammonia too. I am expecting it to disappear, but as I have warned earlier, if it remains and the pH rises above 7, danger. So go slow. Try to do the WC's at roughly the same time of day, and this should preferably be early rather than late; disrupting fish in the morning to give them time to settle before dark is preferable. Also, pH should be tested at the same time each day because the normal diurnal fluctuation willnot impact the readings.
If this goes as I would expect, the ammonia will disappear and the pH will slowly rise. Once the ammonia is gone, back to once a week water changes, 1/3 of the tank. Again, test pH just prior to the WC until it seems fairly stable week to week.