You are correct about stability, has nothing to do with pH whatsoever.
First of, you need to figure out what the actual pH of the tank is. Test strips are notoriously inaccurate. Even a cheap API test kit should be much better than a strip. If you have a good local fish shop, take a sample of water to them and see what they test it as. You also need to get an accurate measurement of the pH of your tap water. Test it straight out of the tap and let it sit overnight and test it again. The main concern here is if your tap water is significantly different than your tank water. If this happens to be the case it would be a good idea to try and figure out why there is such a large difference and make plans accordingly from there.
Unless your pH is extremely acidic or basic, it's best to just ignore it unless you are trying to breed a specific species. pH in a tank is variable mainly because of its correlation with CO2. Algae and plants will consume CO2 when they are illuminated which is why pH goes up during the day time. Have multiple people in a room and you can measure the pH drop in a tank. Opposite thing can be said if you open a window or turn fans on in the house.
Example, this is from my main reef tank.
You can see the ph swing throughout the day, but on the 21st, we turned the AC back on which reduced the CO2 concentration in the house and the overall pH went up with it. It's just such a moving target that it's better to not mess with it.
Don't try to buffer it and don't add buffering agents unless you know exactly what you are doing. At best these are a temporary solution as they aren't permanent and will greatly increase the maintenance on the tank as well as having to pre-buffer the water during a water change.
Most fish can thrive just fine in lower or higher pH than what is their ideal. I keep discus in pH 9 tap water and have never had a problem with it. The main thing that all fish need is stability.
You nitrates are fairly high. Based on the specs you listed, this is what I would assume is the problem. Also, if these fish were newly added or this is a new tank, it wouldn't be uncommon to have losses even if you properly cycled the tank. This is subjective, but I would start doing 50% water changes per week, always make sure to treat the water for chlorine or chloramines, and see where your nitrates and fish health are in a month or two. Reduce water changes once you get the nitrates in control.