You have access to a tds/ec meter? A ppm on res would tell a better picture .
But as all suggestions state, it’s likey a nute burn. However, it also curiously looks like russets mite, that rusty look to leaves, can we get a pic of the affected leaf underside?
But likely a nute off balance. Every hydro grower would say, what’s the ppm? Reason is that no matter what the notes on the nute can say, true pic is via ppm. Plant life stage has different levels on ppm feed and more accurately done via ppm additions. The tsp guides are very imprecise so I’d lower that to half and let plant recover then pls get a good meter. About $170-195 for a decent dual handheld or $340-500 for the pro model but the hand heads are precise and most hydro stores will calibrate the meter they sell for free once monthly.
No panic, just cut the feed in half. Check for pests, but likely a nutrient issue. The purple stems usually is a dead giveaway for over feeding or wrong dosage to plant life stage. I see the purple when I switch from veg to flower nutes, usually for a week or so until the plant realizes it’s time to go to a different life cycle. Have you perhaps added the wrong feed at the wrong cycle? Hard to diagnose via pics, safest best method would be to cut what u do in half and monitor.
Like some one said it’s a weed but dang it she is temperamental!
Cheers and pls update us! Greet setup though brother!!
