I had this happen to me recently, you really have to think about what makes sense here... I started researching plants and nute problems in general, aside from cannabis. My brown spots showed up and I initially thought it was some type of a deposit in the leaves, so I flushed and assumed there were still nutes in the soil. WRONG, once my leaves started to yellow I knew it was a N deficiency, and the brown spots were a P and/or K deficiency. SO i prepared a weak solution and sprayed the leaves the next morning before turning my lights back on. I pruned the damage leaves to rid them of the spots and see if anymore showed up... within 3 days the leaves were green and no more spots appeared. I tend to under fertilize the first few weeks and get a feel for my new strains ability to absorb nutes. they all work so differently it amazes me. Even within the same strain each plant has its own regimen. Hope this helps. Just remember to give them a little shot of nutes every time you water (2-4 days apart). Mainly macro nutrients are the ones that need replenishing more often than you think, but don't overdo IT! be gentle!