Hard to completely diagnose and provide advice with the information given but I will try to help.
RewisL19 wrote: ↑Mon Jul 18, 2022 10:59 am
is this a magnesium deficiency?
Yes with the evidence being red veins and red main stem in picture 1, however as kbag mentioned there's more going on than just a simple mag def.
Observations I've made.
8 sad plants, it's just one looks worse,others soon will follow.
red pinstripe main stems.
red leaf veins.
serrated leaf edges curling (canoeing) upwards.
bottom leaf sets show calcium def.
generally over watered.
generally underfed.
Id focus here:
serrated leaf edges could be high temps but I suspect your lighting is too intense.
Everything else I've listed above can be solved by changing your EC and watering schedule.
What I would do
Id dim my lights or if you can't then raise them up.
Feed and watering.......Firstly allow the pots to dryback to 50/60% then at this age I would feed the following to 20% runoff. A+B 1.0 EC / tiny amount of epsom salts for extra mag 0.2 EC & finally canna calmag at 0.3 giving me a total 1.5 EC. Bear in mind I use RO water so you might not need the canna calmag.
Now Id leave them alone for 4 days with no further waterings, lift the pots feel weight & if needed repeat the above feed again. It's important to let it root out.
I would keep repeating this (feeling how heavy the pots are etc) until I was feeding everyday then i'd switch on the automation.
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like i say its hard to say for sure it could be something else (environmental) but I've gone with what's most likely given what i see in the pictures, I think if you follow this advice your plants will be green in a week.
Let us know how you get on