Whats causing this ?

This is my forth attempt at growing and the same problem keeps happening, the older leaves start to go pale and die. here are some pics

I'm growing in a space of 60 x 100 x 130cm
1 x 125w envirolight 6400k ( main light )
2 x 30w 6400k bulbs ( extra lights on side )
100mm RVK extractor fan
tower fan
I'm growing in plagron light mix with a ph of 6.6, watering with 6.4 ph'd water and the run off is 6.6. As this has happenend on each grow at first i thought it was a lack of Nitrogen, but i'm pretty sure its not. I've tried different compost Bio Bizz now Plagron and different nutes Bio Bizz, Earth Juice, etc and have not been able to stop the leaves from dying. Would epsom salts help, are they getting enough light ? Please help !

bigger pots bro for the first thing and some soil will do that i had it going on too i dug up some garden dirt and transplanted them in that and it went away i was using grade A soil nice and black had the same thing as you did after i put it in what i call crapy soil BAM thy took off.

yellowing well into flowering is totally normal, you could give it one feeding with a boost of nitro, like fox farm grow big. i give mine grow big 2 weeks into flower then switch to tiger bloom the rest....its good to give them a boost of nitro right before flowering, regardless though the first thing to die when you 12 12 em is the fans...how long has it been in 12 12....also you dont give the babies nutes right?

i just harvested some mass super skunk and it came out awsome, the coolest thing was that in the end it came out purple red, dark green orange and yellow. it was a very cool looking finisher.