Having seen this pics, my money is on a micro nutrient deficiency in its early stages.
If you live in a soft water area, some cal mag may be needed but only as a last resort. Most likely a feed to supplement what the plants are getting from the soil is needed.
If you have not fed them, it may be an idea to give them a 1/4 strength dose of some grow feed.
They are nice looking plants, you are doing well so far.
Hope this helps.