Yep, same thing happened to me with the MG soil. Mine didn't happen until after 3 months, tho. It had the 3 month supply of nutes. I gave it a boost with all purpose miracle gro nutes, and when that ran out, used some miracle gro fish emulsion (5-1-1). I had yellowing leaves, the whole bit. Now that it's flowering, some yellowing is normal. The 6-12-6 nutes you plan on using should work fine. Just keep an eye on it. But from here on out, you will have some natural yellowing of leaves. Best of luck to you.