Sounds like your deion resin is spent. If your filter is removing 99% of 400 tds then youd have 4 tds at the output. Everything sounds good to me.
You probably have high co2 in your water and deion resin hates it and dies earlier. Well water gives high co2.
You probably have high co2 in your water and deion resin hates it and dies earlier. Well water gives high co2.
