Hi man, hope you're well? Is this happening on all plants? I am no expert as we are all still learning. The learning is the best part I think. From the look of the leaves it could be a magnesium or sulphur deficiency. Easily sorted. But some Epsom salts and dissolve about quarter a tsp in 500ml of water. Use as a foliar spray. Helps with nutrient uptake too. Epsom salt are basically magnesium sulphate. I hope this help, if not sorry for talking shite... ✌️