Rusting leaves?

I agree with @gottagrowsometime - I don't like the look of that either - but I don't think it's a fungus (which is, I think, where his mind is going)... I think it IS a calcium deficiency but you don't have any information on nutes that you're giving her other than telling us you've added some calmag .... calmag needs to be used judiciously - and only once a week... you'd have to wait until you have some new growth to see if the problem persists or if you've resolved it.... the damaged leaves will not get better - but if the calmag is working, they shouldn't get worse. I'm more concerned, I guess, with your PH. You SAY it's 6.8 but how do you measure it? Do you have a PH pen? Have you calibrated it lately? If you've got a grow shop somewhere close, it might be worth taking your own PH reading of plain water then take that same container of water into the grow shop and ask if they would measure the PH - just for comparison... The reason I ask this is that the PH SEEMS right and your plant shouldn't be having any trouble uptaking calcium (calcium won't be absorbed if the PH is below 6.2 or above 7.0). If you're only measuring with strips of paper or the drops and then comparing to a printed chart, those are wildly inaccurate... my advice would be to invest in a PH pen if you don't have one as it is a critical tool that will prevent a lot of problems in any grow... Good luck...