The first thing I would check is the pH of the water you're using to fill the tank with.
You've said that you tested it already and it was normal, but it could actually be that the water is normal as it's coming out of the faucet and then later reverting to the state it was at before. The water parameters could be affected by your plumbing, or by your local water municipality if they have certain parameters they're expected to maintain.
Note: Even if you haven't had problems with your water in the past, it's possible your local water municipality could have changed or upgraded their equipment or methods, or simply changed their parameter requirements. That's why it's always a good idea to perform tests on your aquarium periodically to ensure that everything remains the same.
To test if that's the case, I'd suggest testing the water after it's been sitting in a bowl or a bucket for the amount of time it takes your tank's pH to change after performing a water change. Typically I think it takes a day or so before you start seeing anything noticeable.
pH is affected by a couple different things, but what I think is changing it in your case is probably the oxygen and carbon dioxide levels.
I know pH will rise if either carbon dioxide is taken out of the water or oxygen is added to it, and lower if carbon dioxide is added or oxygen is removed. So what I'm guessing that's what's happening to your water. Perhaps there are organisms in the water consuming oxygen and releasing carbon dioxide over time.
Some ideas on how to keep a higher pH in your aquarium:
The pH will typically lower over time eventually, that's part of the reason why water changes are important, to keep a stable pH level. But as your pH levels are changing more rapidly than what I'd consider normal, I think that it would be safe to consider adding things that will work to keep the pH levels up.
The most common method I think is using either crushed corals, or limestone. You can use them as a substrate, or place a bag filled with it into your filter.
The crushed corals would be calcium carbonate, which when mixed with water form calcium bicarbonate, which will absorb carbon dioxide in the water. Limestone is formed by the minerals calcite and aragonite which are different forms of calcium carbonate1, so it acts in the same way.
Note: This is different than carbon filters, which use activated charcoal to filter out chemicals and colors in the water.2
Otherwise something as simple as gravel substrate can help buffer the pH levels to become higher. Likewise, adding driftwood will help buffer pH levels to become lower.
Adding plants will help maintain pH levels as they absorb carbon dioxide and release oxygen into the water. Although if I remember correctly, parrotfish like to eat plants like candy, so it might not be viable in your situation.
If nothing else, there are chemical additives that help raise, and lower, pH levels. I've used API's pH up without any ill effects. Of course use it in moderation so that the pH isn't changing drastically.