Nutrient deficiency??

First of all, you need to start a diary so we can see what you've been doing/not doing and giving/not giving and what all the conditions are. I had a very similar situation with my first DWC - and that was the result of the pH being too high, great pH fluctuations... it was a mess for a long time until I figured out how to control the issue in DWC.. I thought my plant was a goner but she just kept looking at me saying "I don't care what you try to do to stop me, I'm going to keep growing" ... so don't give up on your plant.... but start a diary because that's where all of us want to turn first when questions come up... Good luck...

Drowning from sitting in saucers of water for too long perhaps.... Maybe pH issues.... Maybe over feeding/too hot soil mix.... Maybe a combination of all 3? Without a diary and seeing what you are actually doing, makes it hard to offer suggestions any more in depth than random guesses really.

How to Use Hydrogen Peroxide to Treat Soil Pests and Root Rot Mix one part of additive-free 35% hydrogen peroxide with 10 parts water. Water infected plants thoroughly. The soil will bubble as the oxygen is released. For pests, water with the mixture twice a week, allowing the top 2 inches of soil to dry between watering. Root pests should subside within a week. For root rot, water plants very thoroughly, then allow the soil to dry. The top 2-3 inches of soil should be completely dry before returning to a regular water regimen. If the process is done correctly, root rot can easily be treated with only one watering of peroxide.

Soil looks dense and lacking oxygen, moist soil that stays wet will lack oxygenation required. You just be careful not to overwater using old school pots. Hava a fan blowing air directly on the plants but gently. Set your tent ambient to 80 , drop light until top of leaf canopy reaches 85 but no higher. Looks like rot has set in tbh, might not be too late. No more water please. You should look to buy "stepwell supersoil" so long as you go 7gal or bigger you never need to feed period and you will always have perfect soil. 100% Organic.

PH issues i reckon. Hard to say without a diary.