I’m gonna suggest that both test kits are faulty. If it was a liquid test kit the reagent #2 bottle needs to be banged against something solid as well as shaken.
Wood will only reduce the pH in very soft water. At a pH of 8.6 it’s unlikely to be soft.
Is the filter media porous? If so it’s possible that it contains anaerobic bacteria that are consuming the nitrates.
Wood will only reduce the pH in very soft water. At a pH of 8.6 it’s unlikely to be soft.
Is the filter media porous? If so it’s possible that it contains anaerobic bacteria that are consuming the nitrates.
Heterotrophic bacteria produce ammonia so there would still be nitrates. Also the tank inhabitants would not be more prone to disease if there are no nitrates. They would be less prone to disease. Nitrates are a stressor and the ideal level for shrimp and fish is zero.