Strawberry lemonade

I think it could be possible that your lights are a little to far away from the plants.

I guess a lack but you have to know how high is your ec and how high has it been in the last few weeks?

Looks like they're possibly "under-watered / root-bound". (small pot size for a long time. (this is how I started off growing!)) What are your watering/feeding habits? With a (small) pot full of roots, you will want to water so that some liquid flows through the roots, and out from the bottom of the pot, ensuring full saturation. - Make sure to remove the excess water from below. Then allow the soil to dry out before watering again.(2-3+ days usually) - "Flood and Drain" If this isn't the issue, then it's likely just the plant shedding under-lit leaves, as the plant matures towards harvest.