I have a 4 stage RODI. I test it regularly with a handheld TDS meter. It is running at 0tds. I wonder if the chaeto is using up phosphates and nitrates, which is contributing to an increase in Alk.I was having this issue as well with instant ocean salt it was mixing at 1.026 salinity and dkh at 16 but I used tap water + instant ocean . ( now use only lfs saltwater as mixing salt creates too much for me , 1600 Mg , 500 Calc and 16 dkh)