I honestly can't say if this is an issue or not but I personally would try losing the "apple cider vinegar" and go with true ph up/down from a hydro store.

What's low grade professional soil? Never mind...I'm sure Stinkster knows.
Weedhound Reviewed by Weedhound on . Yellow Leaf tips... Please help! Plant seems healthy and is growing but all new leaves have yellow tips! Medium: Low nute "professional grade" soil (.09, .04, .06) mixed with sand and perlite - 2:1:1. Lighting: All CFL - 1-125w (6500k), 2-42w (6500k) & 2-42w (2700k) = 293w CFL total. About 3-4" from plant tops. PH: Soil - 6.8. Water - 6.8 (tap water - adjusted with apple cider vinegar and left out for 48 hours before applying to plants) Temps- 77-84. Thermometer with remote sensor positioned a little higher than plant Rating: 5