Lower Leaves Yellow / Brown/ Rusty / Dying / Dead Help!

I know lower leaves go yellow and drop off but these littluns are only a few weeks old so it doesn't seem right.

They're in Bio Bizz Light, watering twice a week, fed 1/4 dose of of Bio Bizz Grow once a week, 600w HPS, Temps are 75-85, Runoff is Ph 5.8-6.5 and Humidity is too low at 35% RH (working on this though).

First appeared before first feeding, took this as a sign to start the nutes but it has worsened since then, new growth seems good but leaves seem to look less healthy the older they get. Am thinking it's a deficiency of some kind. Any thoughts / diagnoses?

Keep in mind all your upper leaves and new growth are looking real good. You might have done something early on that shocked the lowers temporarily.

Best thing, don't water too much let your pots get light before the next watering. Make sure you got good pH control. And give the soil a balanced diet but don't over feed.

Your in veg so give it a balanced NPK and make sure they are getting CaMg.

I always swear by some light algae tea, it's loaded with trace minerals and decent PK.

But like I said the tops of the plants are looking good man!

Inspect the leaves, possibly somethings chewing on the underside. Also possible it's a virus or blight. You may want to consider trimming those leaves to save the plant, but be certain you need to before you cut. I don't like to trim the leaves unless I have to. Those leaves might not be doing to much good now anyway they're in the shade and they're dying.

Thanks man, glad the top are looking right. I have been making sure the soil has dried properly before watering, have been cautious on that front.
I think you may be right in that I shocked the plant early on though, all the effected leaves were sprouted before the first feed so I think I may have starved em.
the Bio Bizz has the NPK covered now but need to look into some CaMg suppliments.
No sign of bugs but really hope its not a virus! Will removing the leaves stop spread if so? The worst ones are crumbling anyway, don't think there's much hope for them really.

mine are doing the same thing but thats because im using miracle grow slow release soil and it shocked her as a seedling and i had to flush it a few times

Mine started turning bad before any nutes, don't think I could have burnt it. I'm thinking maybe I left it too long and it got a N deficiency?