Its not nute burn (pretty positive). pH is stable at 5.7 with doser. no ozone. Already treated for magnesium deficiency (with epsom; 1tsp per gallon) at first signs of rust spot five days ago. Gotten worse since. New growth flourishing. System is DWC. Nutes are lucas method (10B-5M) plus liquid karma. Using RO water. Calmag and sensizyme on the way. Could Calmag be the answer to this problem? Sorry pictures are unavailable. Spots are on older leaves and are all inbetween the veins. Very typical of magnesium deficiency, could the epsom just not be working yet?