No matter who said what before, I'd be very concerned about that well water. Exhumed, I'm sure you know that if you have higher than normal nitrogen levels in your well then they are leaching in from the surrounding soil. Obviously what is happening to your fish is not normal in this hobby. And well water is notorious for having high nitrates from farm run off. Must be why your planted tanks do so well. I would take this seriously and want the issue resolved for my own health. If you have nitrates coming in, which are harmful in and of themselves, then you don't know what other chemicals the ground may be leaching. My point is its not normal and shouldn't be shrugged off as only harmful when under pressure. It could be very serious in the long term.
My brother in law actually had his well water tested for nitrates before their new baby was born, as high nitrogen is associated with infant deaths from basically the bends. Turns out they were coming in high, like 50m/L. So they installed an aerator that brought it down to zero. Dissolved gas may be the issue but the source is that well since they are susceptible to just about every environmental shift that occurs.