What's the best way to correct the deficiency...

Hey there GreenT, you have a magnesium deficiency, you can tell from the yellowing between the veins of the older leaves. It's pretty common because magnesium is used to regulate photosynthesis and because you have an intense light schedule , the plants simply need more magnesium than they are able to absorb from the soil. To correct this, add cal mag to your feed every week and spray with a cal mag solution (something like 3-5ml/L) two to three times a week and the problem should stop spreading and the plant will fully recover within 10 days to 2 weeks. Hope this helps ! 🚀