This is due to all the "bacteria" you are adding with the Stability. I put bacteria in quotation marks because it is not the true nitrifying bacteria, but it does according to tests somewhat help them establish a bit faster. Stability does seem to work, somewhat, I used it once in a problem tank. Anyway, this plus the tap water nitrate is the cause of the nitrates.
If you have live plants, and they are showing signs of growth (not dying off), and some are fairly fast growers (surface plants are ideal for this), you should not be adding anything else (other than water conditioner at water changes. You will still see some nitrates due to the source water, but they would be lower than what you report. And BTW, aquarium plants do not use nitrate. They take up ammonia/ammonium as their preferred nitrogen form, and they are so fast at this (if rapid growing) they outcompete the nitrifying bacteria so the ammonia/ammonium is taken up but nitrite is not produced, and thus less nitrate (some heavily planted tanks have zero nitrates for years).
On the pH issue inn post #1...the CO2 dissolved in tap water can cause a false (low) pH reading. Out-gas the CO2 before testing tap water. Do not need to do this for tank water, just tap. That will give you a more accurate reading of the true pH of the tap water. Let a glass of fresh tap water sit 24 hours to outgas, or some will agitate the water very briskly for a few moments.
I don't see the fish species mentioned, but 80F is way too warm for most fish (there are a few exceptions). This is stressing the fish, as temperature drives their metabolism and thy must work much harder just to maintain normal bodily functions. And never turn off air to an aquarium, especially here, that adds more stress as you seem to have noticed. Things like this only stress fish more, weakening them, and that allows other problems to emerge.
Last comment for now, on Prime. This is not the "saviour" many assume. It does not remove ammonia, nitrite or nitrate, it "hides" them for 24-36 hours (maybe) but they remain. There have been controlled studies carried out on Prime, and you might be surprised at the results. A couple links.
5.5.3.2.1. Prime, Safe and Ammonia (aquariumscience.org)
5.5.3.1. Ammonia Detoxifying (aquariumscience.org)
Fresh water is of more benefit in the long run, certainly when live plants are present.