The plants are in big trouble. The soil should not be green. You are using two different soil mediums, which is a bad idea. The roots have probably not grown out of the small inner cube and as a result the plant is dying, while the roots are getting burned by nutrients. Probably acidic conditions, 5.8 is kinda low. Honestly, I don't want to sound rude - but I think everything is wrong here. Soil medium, nutrient solution, PH is too low, probably lack of light and nutrient mix might be inadequate. Biggest problem is DEFINITELY the rooting medium. It doesn't look right in the picture, you'll need to move the plants into something else.