At first glance I thought phosphorus deficiency, but not 100% sure. Make positive sure the pH and water temp is in range. No other plant is affected so I'm thinking it's individual buckets. I've only grown dwc a couple times, if I got to where I couldn't figure out the nutrient issue and correct it I'd dump fill back up fresh. I'll give you credit growing like that, for me it was just to much hassle to keep up with and and now stay soil or coco.
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Looked all over,none. The 4 other plants in the tent all ok,same food and
Have you checked your plants over good for pests?