Yellow Spots

started with cuttings in rockwool about 6 weeks ago
upon rooting placed in organic potting soil
after established, transplanted to larger pots with 6-month miracle grow (all my grows are miracle grows and that's that )
the only other thing i do is water them with de-chlorinated tap water when they go dry. i try to water a day before they go limp
normally they get one more transplant into larger pots but it is too early for that
so anyway, i have some yellow spots
i have an ongoing knat problem, but it is minimal at the moment (actually, haven't seen any at all this grow)
could be magnesium maybe, i do have some purple stems high on the plants
any thoughts would be appreciated

It look like they're hungry I would start feeding them 1/4 strength nutes and add some epsom salts about a teaspoon to a gallon at first then work your way up with both...good luck...

If they still yellow it's light burn !
=>Find a way to lower the temperature and/or increase the circulation in the grow room or grow area if heat is the problem. Having a small fan blowing over the tops of your plants will help prevent hot spots from forming directly under your grow lights. If your marijuana plants are just getting too much light, try removing some of the lights or moving your lights further away from the tops of the plants.
When learning how to weed, it's best to try to keep things at a comfortable room temperature at all times for optimal growth. If it's too hot for you, it's probably too hot for your plants.

If they turn brown a little bit : Calcium Deficiency
=>If the pH of your root zone is off, then your cannabis cannot properly absorb calcium through its roots so the first step is to ensure that you have the correct pH for your growth medium. Different strains of cannabis tend to have different nutrient problems, but calcium, magnesium, and iron deficiencies often appear together in cannabis. Therefore many growers decide to purchase some sort of Calcium-Magnesium (often called Cal-Mag) supplement for their grow room in case this common deficiency appears. I have listed some available Cal-Mag supplements that I have used along below with some general information about each one. After supplementing with Cal-Mag and correcting the pH, you should expect to see new healthy growth within a week.