Help i think my plants are slowly dying!!!

Hi there, im having a bit of a problem with my plants (see pics) About 4 days ago i cut all the dead leaves off all my plants (there was only a couple at the time) and they all seemed healthy, until today, i have noticed that quite a lot of the lower leaves are really yellowing, covered in brown 'burn marks' and ultimetely dying (as a few already have)

The plants are about 50cm tall in Veg, 18 / 6

Theres a few different strains and its happening to them all.

I have a 2.4m x 1.2m tent, 6" intake and outtake controlled by a primair temperature controller.

2x 600w HPS over 6 plants.

They are growing in Coco with clay balls in the mix for added aeration, in 20 litre pots.

I am currently feeding them Canna Coco A & B Plus extra liquid nitrogen all at the full dose.

Water every 2 days, 3 feeds of Nute Mix, then 1 feed of just water.

PH is always 5.8 - 5.9

Using tap water but its left open for 24 hours

the temp is usually around 28.3'c, but with the recent heatwave its been 31'c most of the day

The lights are about 20" above the plant and growth seems good tbh, my main concern is that all these leaves at the bottom seem to have just gone like this overnight.

anyway take a look at the pics tell me what you think.

Honestly I dont think they look like they are dying. They do look hot though. I remember being told pure coco should be watered everyday. But I've never used the stuff. Also 3 days in a row of feeding seems a little much. They look really dark, you might be over doing the nutes a bit. Those yellowing leaves arn't something I would worry too much about. In fact you should spend a few hours in there thinning out that lower canopy of all that thin growth. It will improve air circulation in the room and make the plants focus on their main budding sites.