I'm not clear why daily water changes are needed at this stage... but water changes are beneficial so no real harm. If you are testing ammonia and nitrite and either is more than zero with the API liquid kit, then a daily water change should be done.
There is no question but that the monthly water change and related cleaning was causing a significant alteration in water quality, which could have been any one or more of the ammonia, nitrite, pH, nitrate.
You mentioned pH being normal, but we don't know what this may mean. What is the tap water pH and what is the tank water pH? And when testing tap water, put some water in a jar with a lid and shake it briskly for a few seconds before testing. This outgasses the CO2 (carbon dioxide) which can cause a lower pH reading.
Do not use any products to deal with algae; these are chemicals that will affect some plants and fish.
Never use pH adjusting products, as the pH is the result of a complex chemical process and these products are highly stressful to fish.
Knowing the above numbers will allow us to sort this out.