Brown Edges on lightly yellow leaves

[Niggy Tardust]

I am not sure what is going on with my plants. This is the story so far. I am growing in coconut coir under 800W MH light on veg day 69. This was supposed to be my first day of flower. I am running at about 76 *F with constant airflow. I have a CO2 system running every 45 minutes for 15 minutes. Everything was going great until about 1 1/2 week ago. The plants started showing some signs of nutrient lock. So I took all the plants and flushed them out. I let them sit for a few days to let the medium dry a little before I fed them. The new growth started comming out a little yellow and twisted at this point. So I fed them and dosed them with epsom salts (1/2 cup per gallon of H2O, as per instructions on package). Now, the fan leaves are brown around the edges and some of them are drying out completly and dropping off. My lights are a good 16" away from the tops of the plants. My CO2 PPM is at about 1000 which is in the range for healthy growth. I am using earth juice fertilizer (Grow, Microblast, Meta-K, Boost). I am growing in 3 gallon pots. I checked the root structure to make sure they werent root bound. The pH of the soil is right at 6.7 in all of the plants. They were doing so well until as of late. Please Help Me!

Thank you all for your time.