Older leaves are yellowing, brown/red rusty spots, dying tissue, red stems.

PH is fine well within the range for plants to uptake nutrients. ....[][][].... It looks like it had some Nitrogen toxicity but if the runoff is 0.8EC then it is now gone. .....[][][]..... If this is from your diary of Biscotti Feminized, sugar royal + CaMg and Silica is not a complete feed, it needs something with a more rounded out profile. all the macros and micros are needed when growing in a soilless medium like coir or peat. You need to still use grow and or bloom nutrients and if they don't have micros in it add in a micro based nutrients. ....[][][].... from the looks of it it is deficient in Ca and maybe some K issues. Also might be a small amount of micro deficiencies . Fix the feed and give it a good water with min 10% runoff. ....[][][].... Good Luck!