It's not NEW damage though, just a continuation of the original damage.
Not only is it the necrosis, but the leaf serrations are deformed.
What KIND of miracle grow is it? I dont believe the newish organic miracle grow that comes in a black bag should be too terrible. If that's the case, it may just be an issue of the soil being too hot for a young plant.
Also, what is the pH of the water you are using? That could be part of your issue as well.
And as far as soil (when repotting), you dont NEED foxfarm ocean forest or anything else super fancy. I'm not sure what you have available locally, but if you cant find an organic potting mix (preferred), you can usually find topsoil, organic compost, and perlite at your local hardware store. Mix 25% topsoil, 25% perlite, 50% compost... add in a handful or 2 of earthworm castings into the mix, and that should get ya started out well.
As long as you have quality nutrients to use as additives, not having the highest quality soil is more than manageable. I use biobizz products, but getting ready to try megacrop which I'm seeing great things about. Dynagro and General Hydroponics I've also heard good things about (if you need nutrient brand suggestions)