The exact same thing (yellowing of leaves between the veins) happened to me using Freedom Farms soil. I would pH my water, and then get rid of the Chlorine, by using an air pump. It looked like a Magnesium deficiency.
Then one day I tested my water, and the pH was soo high, off the charts. It turns out, bubbling water raises the pH, because it gets rid of the CO2. I had to re-pH my water. I was getting nutrient block out, hence Magnesium deficiency.
Why did it take me so long to learn this. I hope this helps somebody else.
