Top leaves = chlorosis // Bottom leaves = green

I don’t think not having a bag of Co2 this time round would do this. Unless you are still running very large lights and high temps like you would have to have done with the Co2 to get any extra from it. So yes, if you are blasting them with light and heat and no Co2, this is probably your issue. But if you are at regular temps with a normal grow light then it’s probably nothing to do with Co2 as there is enough naturally in the air for most grow setups. I would say since it is happening on the top half only that your light may be to close. Try backing it off and see if the new grow up top continues to do this, the old growth won’t repair itself at this point so you can’t go off that. Hope this helps, Cheers💨