I would say that's an Magnesium (Mg) deficiency *curls up, yellow between veins, necrosis, yellow older leaves starting lower* your new leaves don't have the problem so im 99% certain its Mg
Epsom salt can fix this easily by mixing 1 teaspoon per gallon
Also you need to drop your pH from 6.7, Soil pH needs to be between 5.8-6.5
Most nutrients the plants use are soluble only in a limited range of acidity, from about 5.5-6.5 in mineral soil. So, that is most likely why you may be having a Magnesium deficiency.
CO2 dissolved in the water lowers its pH. CO2 forms carbonic acid when it is in solution with water.