I'm concerned for one of my Dark Devil seeds,...

Few possibilities. Pots are too wet preventing nutes from being absorbed by roots. Ph is too high preventing nutes from being absorbed. Lack of nutrients during veg. Low humidity causing a vapor pressure deficit (VPD) which prevents the roots system from absorbing nutrients. Could be all of the above. You may need to add more perlite to your mix so it drains more freely on your future grows. Unclear what type of soil-less mix your using if its peat based or coco ph of 5.8- 6.3 allows for the best nutrient absorption. Lastly raise your humidity levels by adding a cool mist humidifier to your tent. Week 1,2,3 try to keep the humidity as high as possible in relation to the temperature. 65 to 70% is perfect until the root system is large enough. Some times I use a clear plastic cup with a few holes in it when they are small. During veg you can start dropping it down to 50 to 60% and then under 50% during bloom. Look up Vapor Pressure Deficiency in weed plants it will explain why temperature/humidity levels are important when growing and how it effects water absorption by the roots. Good luck hope this info helps you out with your grow brother.