Hi GC, 400w hps @ 1ft away from plants. Temp is always at 80F with light on and never rises above that. 3 gal pots. 2 months old in veg stage about to start budding on the 1st. (All from seed) Using Fox Farm Grow Big & Cal-Mag with PH at around 7 after the nutrients are added. Only using 1 teaspoon of each per gallon. They get watered 3 times a week, they only get Grow Big and Cal-Mag once a week the rest is just water. At the moment out of 7 plants 6 are doing this, one is doing great with no yellowing or spots. The yellowing and spots haven't reached the top of the plants yet only the lower and middle of the plants. Here is a pic of a leaf from one of them.
Is your water at a proper pH? I think you may have given your plant too much magnesium but i could be wrong. I would make sure you flush your plants but make sure the pH is correct otherwise the problem will just persist.