I've got a couple of seedlings (lowryder 2), at around 3 weeks or so. They have both had exactly the same treatment and are planted in Miracle Grow under CFL 18 hours a day. It is a 70watt CFL (4300 lumens) and the growing area is surrounded by a reflective mat with some yeast also bubbling away in there. I water them sparingly and have not added any nutrients. However on one of them the leaves have started to look quite yellow! The problem is not nearly as noticeable on the other plant, which is also growing faster. What could cause this? Pictures attached.
It's my first time growing so apologies if this is a silly question- I did search around the forums first but didn't find anything that looked like my problem.
Thanks in advance!