Hard to say without any readings. But most likely nutrient burn.
In hydro you just add minimal nutes at that age. My schedule is 1ml/gal grow a and 1ml/gal grow b.
I use emerald harvest.
Whatever you use probably has a feeding schedule you can follow.
It could be any number of things. If you are using the drip ring you are going to have problems. If doing DWC how far below the net pot is your water level? Is there an air stone? PH is too high. What is your rez temp?
Work with us, please. Without the info it’s just guessing.
No air stone, water level about 2in below rez pot. pH is currently 6.4, water was changed two days ago. Temp today is 74.5 however a couple days ago temp got way too high, 88.8.
If in DWC the plant gets all of it’s oxygen from the rez water. You HAVE to have air or the plant will drown.
This is about right.
Your PH should be 5.3 to 5.8 for hydro.
This is going to kill your plant, combined with the lack of air stone. Temps should be below 70F or root rot will set in (pythium). If you can’t get it under control you should add a chiller. FYI the higher the temperature the less dissolved O is present.
If you are not running any beneficial microbes like Hydroguard you will be at an even greater risk of root rot.