Doesn't sound like to much, mine are auto's so nute sensitive I got very mild nute burn when I first started nutes and roughly 3 weeks later this problem so leads me to believe mines excess. You could try increasing the feed and see if gets better or worse or give plain ph'd water and see if gets worse. That will tell you if it's excess or deficiency it's hard to tell sometimes because an excess of one nutrient can cause a deficiency in another so your feed more thinking deficiency making the problem worse.