That snake skin could be the beginning of damping off. Be sure you keep the fan gently blowing across your plant. I think 1200 ppm is a bit high at this stage. If your pellets are submerged, think about lowering your water level to let the roots dangle into the nutes. The pellets can wick moisture upward and drown out the plant, so be careful here. If it starts to look pinched, you will need to work very hard to save your baby, but it can be done. How are your light levels?