New pics, halogen and cfl

Hey gents and ladies. I am at day30ish, and one out of my three plants is looking great. Very healthy, and lush green. My other plants are looking good too, except for the fact that at the very tip of maybe one or two leaves has turned brown/yellow. Only the extreme tip, a tiny tiny bit. I am wondering what the cause is of this. I feed them once a week with nutes, SENSI ONE GROW, and I have topped one of the three plants in hopes to branch out more. They are also taking quite a bit of time to grow with these halogen lights, so last week I baught a 150 watt CFL and the results are a lot better, seeing better gains. Thinking about ditching the whole 500w Halogen deal. They work, but are slow. Here are some pics. The bigger healthy plant is fine. The smaller plant the tip of the leaf is brown, it might be hard to see (cell phone camera) It is the leaf pointed south, closest to YOU.

sounds like a PH problem. .. when my PH was 7.8 i had slight burns on the tips .. i adjusted the ph, gave them a nice flush, and presto.. back to green