antman, your plant looks excellent :thumbsup: ... lower leaves almost always yellow, they are the oldest leaves on the plant, don't get nearly as much light, and the plant has been taking nutrients from them, it's whole life ... you are correct, you do NOT need stronger nutes, the tip burn indicates plenty of nitrogen ... continue what you're doing, you have a healthy plant there ... I sit here every day, and watch growers poison their plants with too much fertilizer ... then, when they see a problem develop, they throw more additives at it, worsening the situation ... I always try to maintain just the 'slightest' bit of tip-burn ... with my NCDG, a rule-of-thumb is, 1/2 dose, once weekly ... you are doing very well, keep at it :thumbsup: