Yellowing lower leaves. Should I add more or less ...

Hi there! If your pH is really around 5.0, you're slowly but surely killing your babies! Make sure your waterings are in the right pH range (6.0 - 6.5). We're missing some crucial informations here : what nutrients have you been giving to your plants from the beginning and at what frequency? Do you check your Ec/Ppm levels every time you feed them? If so, what was your last measurements? What's the distance between the light & the plants? As an emergency move, I would increase this distance, flush the roots for a couple of days with correctly adjusted pH tap water (and maybe some stress-recovery product like Biobizz Alg-a-mic), carefully remove the dying leaves and see what happens ... Also, if you're able to lower down your temperature by 2 or 3 degrees, they might recover faster. I'll be around, good luck with these babies!

It's not nutrients deficiency. Your pH is a little low, try upping it to 5.5.
As for nutrient strength, how can we know if you don't post any data regarding what, how much, and EC/PPM levels of what you are feeding.