2x Sour Diesel Auto + 1 Random Unknown

I've been having issues with one of my plants, now the second one has started showing signs too! I have two autos in separate 20L solution bubbler buckets. One is producing brown spots on the leaves(lower leaves first) which eventually spread. They both get fed the same, the only thing I notice about the problematic plant is thst ppms is slighty higher, maybe by 50ppm and it seems to rise in pH faster than the other one. I have air stones, res temp is 18-20celcius, I use a Migro 100 COB light which is 12-14" away from the top of the plants. Using RO water (4ppm) and like 10% recommended dose of Canna CalMag, Plagron Hydro A and Plagron Hydro B. Ppms don't change very much (maybe 1-5 ppm a day in either direction) so I'd say they are stable. I have seen that and stable ppm and an increase in pH means the plant is feeding and its a good thing. I'm thinking my issues might be with pH? Possibly locking out boron or Manganese? or Cal / Mag? Or could it be that because I pH down one a day that I'm precipitating out Calcium / Mag? I've also just learned that adding pH down directly to the Res will have an adverse affect on the nutes and can cause localised propagation of the nutes, specially calcium...? Please share your knowledge with me