I'm wondering if someone could help me figure...

Magnesium deficiency. Calcium deficiency causes spots on leaves. Adding magnesium via the roots is far mor effective than foliar application. How do you know your water has lots of iron in it?

You have a mag def. I see your sub is a mix of coco, soil. Not great tbh. If you can apply a foliar spray..150-200ppm of Epsom salt added to a spray. And heavily dose the plants.. (Turn your lights down or apply just after lights out. The plant still eats & takes in nutrients dot 2hrs after lights out + it'll be absorbed in the leaves) Also can be applied to sub.

Foop isn't soilless/hydro? so, i'd wager your pH is too low. 6.5-6.8 is better for soil. I doubt that is causing interveinal chlorosis, though. Funny you mention Fe, because that's one of the potential causes. It's not Mg because there are no spots. Ca does not cause interveinal chlorosis. I think you are locking out one or more trace elements - Fe, Zn, Mn... Excess boron could do it, but it's rare to cause a trace element issue with typical ferts and soil. A leaf chart can verify everything said here. Leaf symptoms are indiscreet and require cross-referencing / more info to weed out the less likely options.

Hello! Some more information such as stage of growth of your plants and light conditions would be helpful for a better answer. I would try some Cal Mag. Just add the minimum suggestion of whatever product you have or purchase.