any idea what's wrong here?

first grow, 13 weeks in after a few week stall at the start, soil and GH nutrients + calmag, any idea what's causing my leaves to yellow like this? it seems like it's only the older leaves, new ones are fine. I think it might be because I've been using cold water from my tap (ph'd to around 6.5) which I read isn't good for the roots so I'm going to start leaving the water out for a day or so before my next water, could that be it or is something else wrong?

When using tap water it’s best to leave it out for 24hrs to let it dechlorinate, this way it also allows the water to go to room temp.

For your plant, It looks like a magnesium deficiency to me, but without some extra info it’d be hard to say for sure. What’s your feeding schedule like? Temp day/night, lights/distance etc?

I've been feeding every 4 or 5 days, 3 feeds with GH flora nutrients, using the chart here https://gh.growgh.com/docs/Feedcharts/GH_FloraSeries-DTW_03216am.pdf then 1 without. Temps in the tent are around 26 during the day and 18 at night, humidity around 40%. and a 320W kingbrite LED that I have around 16" away from the top of the plant right now.

Sounds pretty good, When using bottled nutrients you can get a salt build up in the soil that can cause these sorts of issues, do you flush your soil on the day you water only and check the ppm?

Also, Do you feed full measurements listed in the GH feed chart, or half dose? Companies usually put the max dose on those lists to sell more of there product faster, so it’s usually recommended to feed at half dose and up the amount depending on how much she needs as she grows. Feeding more than she needs can cause a nutrient lockout and the plant is unable to uptake that particular nutrient. Not 100% sure if that’s what it is in your situation, but it’s something I’ve run into myself.

I don't flush on the water days - just regular water - or check ppm, I'll try that next time. And I'm using the full dose (3 late growth) of the GH chart but after ramping it up slowly so maybe I'll lower it back down as well, thanks