Yellow leaves!

My first try would be to add 1/4 tsp of Epson Salt into 1 Litre of water. If you don't start to see recovery in 2-4 days it might also need Calcium or it is something else. This is the easiest and fastest test. Your plants are very light green so It could also be that they never had enough Nitrogen and now you are seeing the effects. Running Humidity lower than 50-55% will also be hard on the plant and potentially cause drying out of leaves and light burn. Look VERY CLOSE at the Pistils if any of them have browned or the very tip is looking shriveled then light is too close, too hot, too low Humidity.

Since your into Midflower the plant is pulling needed nutrients from the bigger fan leafs for the flower sites development. You can give it a mild does of some Calmag to slow the fade of yellowing leaves.