High nitrates in well water is usually from agricultural runoff. Basically fertiliser runoff soaking into the ground water and contaminating it. You need to be careful and don't drink water with ammonia, nitrite or nitrates in because they can cause cancer. Don't use it for your pets either if you can help it.Well water have high nitrate this year, sometimes over 85ppm.
For some reason though, (yes, my discus tank is still here)
this year, discus seems to grow faster, tetras became more active and I haven’t had any fish affected and death by diseases other than some newly purchased zebra danios from the store, and me accidentally decapitated a betta during water changes.
Do you think there are any factors behind it or was it just some strange luck going on?
If you haven't been getting as many fish there is less chance of introducing new diseases so the fish would be healthier and less stressed.
Are you growing plants in the discus tank?
If yes, they might be eating the plants and that could be helping them grow faster.
I have killed fish doing gravel cleans too. I had a black moor goldfish decide to swim up the gravel cleaner pipe and got its head sucked off. It was gross. I have had plenty of other fish do similar things too. Tiger barbs and female Bettas were the worst. They would deliberately go inside the tube and get sucked out with the water.