Just a quick update, I dosed sulphate as described above and it is now back up to level, however a word of caution so others don't make the same mistake as I did, as advised my salinity levels will increase as I dose Na2SO4 I corrected for this by removing tank water and allowing my ATO to replace with RODI, however I forgot about the the reduction in CA and Mg that will happen as well, now these levels are low, easy enough to correct but worth thinking about if you have to correct your sulphate levels, not only monitor salinity but Ca and Mg.... now going to use MgSuplhate instead of MgCl so hopefully levels will all balance out