Looks like there are two things going wrong here. One is the yellowing, which could be due to an inability to uptake nitrogen. The leaf curling to probably due to a pH problem. You're using potting soil that specifically says it's for outdoor growing, but you are using it indoors. The ferts in that soil need the bacteria which is in outdoor dirt to break them down to where the roots can take the stuff up. The hot fert load is causing the pH problem and because the ferts aren't being broken down (since the bacteria are missing), flushing may or may not help. Frankly, I think that since they're that big and already in larger containers, it's probably too late to fix this problem. Next time use Happy Frog potting soil you get at hydro grow shop. What you're suppose to do with that soil you have is mix it with outdoor dirt, but you don't want that dirt in your indoor grow. Soils are specially made for indoor growing, like Happy Frog or Roots brand.