I'm wondering if you have too many plants in a small space. They'll get much bigger than that. I'd consider flowering them now. Sacks of pollen will start to show on males. They won't produce bud. Transplant to 5 gallon buckets or bigger. At the end of their life, the leaves will yellow, but only then is it normal. Any time before that might indicate a problem. Go easy on the fert as miracle grow is some hot stuff.