RQS Blue Cheese Autos

The leaves are dying off and there is a little bit of rust appearing on the leaves. Last time this happened I panicked and gave them a load of CalMag. This time I have increased the feed with BioGrow as I think the yellowing leaves denotes a lack of nitrogen. If things don't improve over the next couple of days I'll give them just water for a bit. I'm not sure what the problem is. Perhaps I'm not feeding enough at the beginning, perhaps I'm feeding too much, maybe the light is too powerful? I'm still not sure about BioBizz. Any ideas or suggestions?