Nutrient problems I cant identify confidently as I am a noob

As GM said below, I have no idea what nutes you're giving this plant but based on the look of er overall - yeah. You've either got lock out or a severe deficiency as a result of whatever elements the diy mix might be lacking. you're also in week 3 which means your roots will have hit the res. by now - couldn't help but notice that red tape below the mesh pot line doesn't cover all the way up to the first row of mesh - you might have a light leak issue there as well. I have to use covers over my hydrotons because the light will always get through them and rot out my roots. This has always been an issue for me in the past. If you've got slimy roots then this could easily be the source of your problem. Best of luck.

Diy salt mix? I would suspect this is your problem. You have a nutrient lockout. Too much salt and the roots can't absorb nutrients properly. Or your nutrient ratios might not be correct?