Please do not drink your water until we find the issue, I can't see any possibility of our boiler water backfeeding into your domestic but just in case, it could make you very sick! Can you be home tomorrow if I make a trip up to check it out?
FYI, far as the wife’s aquariums, 0 PPM of nitrites are best, even a trace can have long term effects though, 5 PPM is considered lethal , up to 40 PPM of nitrates is considered normal, lower the better though, if it approaches 80 PPM you have serious issues. Out of curiosity I checked our well water, 0, 0, 0 far as ammonia, nitrites and nitrates. Also zero on pesticides and insecticides, zero arsenic, lead, copper and bacteria.
Unless you have crazy low house pressure I can’t see boiler water mixing with the house water, the other way around of course. I had a gauge once on my system when I still ran the nominal 1” pex, ran about 5-6 pounds on the boiler loop in the basement. However, better safe than sorry.