Help sick plants!!

Miracle grow is fine, you just have to watch your waterings. You're overwatering and watering is when mg releases nutes (resulting in your tip burn). That's not a calmag problem so you should be fine with that for the time being. Reverse osmosis water is not necesarry. I've been using regular water with beautiful crops the whole time. Fairly certain it's a Nitrogen deficiency along with overwatering. If you want to stick with mg, they make N supplements as well

Some one above said MG is time release ferts are water soluble, that's not correct. They do make some potting mixes with time release nutes, however you said you use pro-mix.

Anyway I use regular MG 24-8-16 at 1/2 tsp per gallon plus Cal-Mag EVERY watering. When I switch to flower I use the same PLUS 1/2 tsp MG 11-35-15 orchid fert every watering. Check out the plants in my grow in signature, all grown with that nasty MG. Just put new pics up a couple hours ago.

Don't know if it's pH. You can add some dolomite and cal-mag in your water and it should help raise your pH if that is an issue.