What type of medium; soil or hydro? Soil Indoors or outdoors? Indoors What strain? Northern Lights How old are the plants?7 weeks from seed, 4 weeks into flowering What type of lights and how many watts? CFL - 2x 45W (2700k) 1x 45w(6500k) How far from the lights? 4in What is your watering frequency and source of water? twice weekly, tap water standing for 48 hrs. What, how much and when was it fed? Once weekly, 2ml/l bio bloom + 1ml/L bio grow What is the medium/runoff pH and PPM if in hydro? about 6.5-7 What are the temps and humidity in the room? 20 - 24 Celsius Any bugs? Not that I can see.
Your plant is expressing an Fe deficit... keep your ph run-off at 6.5 and make sure your nute regimen has Fe in the mix. Corrective action would be to flush the plant to lower ph to 6.5 with properly ph water.