That's not spider mites. The blotches are WAY too large. Mite damage presents first as tiny speckles high on the plant, especially in hot grow rooms. If that were insect damage, I would lean towards thrips instead. The pic is a little blurry. Can you steady your camera better so we can get a clearer picture of the damaged leaves?

It's also not Magnesium, which presents as pale striping between the veins, in a very regular pattern that is easy to spot.

You need to be adding CalMag Plus and doing regular soil runoff pH tests if you run RO water. And you need a gallon of medium per foot of main stem. If you suspect rootbound, why are you waiting? Happy feet makes happy plants.