Help with new garden

So I've taken the jump to start my own garden. I built a spot that is 5' high, 6' wide and 6' deep. I managed to get my hands on a bunch of seeds, strains unknown. I know I'm dealing with a bunch of different strains but for now I'm just learning the process until I move to a more solid genetic plant. I'm running 2 HPS lamps, one 600w and the other I think it 800w. I've tested the soil and water and both are a solid 7.5ph. I haven't yet been able to test the nitrogen test on the soil but I've mixed some worm castings in with the organic potting soil for the first batch. First plants are 1mo and second tray is 3days. Light cycle is an 18day and 6night, with no co2 introduction yet. I use a fan/heater to keep the temp in the room a steady 80 during night cycle and just the fan during day hours, temp stays at 89. I've recently got some dieback on few plants, and I'm not quite sure. The closeup of the
4 plants, I'm getting quite a bit of yellowing and a lot of dead veg. I used to run a 24hr day cycle, and I changed to the 18-6 cycle about a week ago. Wondering if the change caused the dieback. I would appreciate any input or assistance of seasoned growers and this is my very first attempt into it.

A pH of 7.5 is too high for normal soil. You want it to be about 6.5, testing the soil itself.

that's a start, thanks!

they are definitely ready to get out of those pots. when the soil is dry pull up on a stalk and the root ball will pull up the whole pot of dirt and leave the pot behind. co2 isnt really needed (unless you can afford to) , just get them some fresh air exchange

I have a fan system set up with a fresh air intake and exhaust that runs for about 15-20min a day to cycle out the air and I'm actually repotting this weekend.