I read through your experiences with your water. I think you’re on to the cause where you talk about anaerobic decay, perhaps even an organism that uses sulfate reduction in the metabolic process, hydrogen sulfide is stinky, poisonous gas that dissolves in water readily. You might try aerating your tank at least temporarily as anaerobic organisms either die or go dormant in the presence of adequate dissolved oxygen. Likely the accumulated biomass at the bottom of the tank is decaying at the hands of a CO2 producing microorganism, the CO2 displaces the O2, opening the door for sulfate reducing archaea or bacteria to capitalize on the accumulated minerals at the bottom of your tank. I hope you get to the bottom of it soon! 😁