I was just wondering if someone could confirm, So...

Hey there auto pilot, so with these water only soils, i think a lot of the time people will typically get better results each grow after the first. If you're planning to re use and reamend the soil. Regarding the phosphorus using the bat guano tea should be the best way because the phosphorus is immediately available unlike some other meals. If you're worrying about the magnesium and definitely the rust coloring etc. Can be a symptom, you can always use some epsom salt which is magnesium sulfate. I'm not sure if you're opposed to using any foliar applications but that can be a great way to not mess with the water only soil but still give a plant a small boost of what it might need, whether epsom salt, kelp spray etc. Just curious but what LEDs are you using, watts? I recently have a few plants that ended up being too close and got some of those brown spots and other things someone might associate with nutrients instead of lighting. Happy growing and good luck with the finish.

Based on what Im seeing I can see some of the purple/red stems this is due to a CalMag deficiency you are right about that , the yellowing how ever that you see is normal . Your plant is reaching the end of its life cycle so its going to try and suck up as much nutrients as it can from the leaves and the medium you have chosen to grow in . With that being said its going to try and suck up all the Potassium and Phosphorus it humanly can giving the illusion of a nutrient deficiency . The reason I say this is because I looked through your grow and it's more less happening towards the end which is why it leads to me to believe that its just the plant reaching the end of its life cycle and not something drastic or something worrisome . Over all its a real simple fix just add CalMag to your nutrient solution and the purple/red stems should start to disappear . If they don't start to go away its a genetic train carried into the seed you are growing sometimes its tricky to tell the difference unless you are experienced with the strain . I hope this information helps shed some light on the issue at hand and helps you to understand what a cannabis plant does at the end of its life cycle as well as how to test if the red/purple is CalMag or a genetic trait . Other then that your ready to rock and roll with some super medicine on the way !!! -Happy Growing!

A magnesium deficiency is usually shown as yellowing between the veins of the leaves. I don't see a PH listed, do you test your ph? It could be as simple of a fix as making sure your ph is between 6.2-6.8. It could also be a phosphorus deficiency that's one of the more common deficiencies during flowering. Phosphorus deficiency causes browning along the edges on mid to lower leaves and purpling stems. That all being said I don't really see much in the pics you posted as far as browning edges and purple stems go. Take a pick of the affected areas under natural or white light.