This plant has been overfed and probably over-watered. There's nothing wrong with Miracle Grow but it is highly concentrated and often people use too much of it. The instructions on the label are intended for plants that are grown outdoors. If you have 1000 watt light or more you can use that dosage but otherwise you have to compensate for your lighting. If you are using a 400 watt, for example, you should only use about 50% of the dosage recommended on the label. If you're growing with CFL's you have to cut back even more. Additionally, that dosage is not meant for every feeding. Use the full dosage only once every two weeks, or even monthly. Between times, use a mixture that is 10% of that amount. (Not necessarily 10% of the label dosage, but 10% of the dosage you use as per your lighting setup.)
You don't need pebbles in the bottom unless your containers have no drain holes. It's a waste of space that plants can use for roots. Always use containers with drain holes and forget the pebbles.
I know a lot of people like moisture meters, but personally, I think they just teach people to rely on a 'device' rather than learning how to read their plants. My own (and many experienced growers) watering method is this: When watering, add until it comes out the drain holes in the bottom which ensures full distribution. Then, wait until the soil dries out completely before watering again. (And I mean completely - just don't leave them sit in dry soil for very long.) In my own humble opinion, if you do these 2 things you will greatly improve the health of your plants, and ultimately your yield.
As for the sick plant, cut back to almost no nutrients for a couple of weeks and don't over water. That is your best chance for saving this plant. (Yield has already been affected, but hopefully you can keep it to a minimum.)