#1. 1000 ppm or even 800 seems really strong for such a small
Plant. I don't go 1000 until about week 3-4 of veg. Never over 1500 in flower.
#2. You may have some disease or virus such as damping off. This seems to plague beginners cause they are always touching and manipulating and generally never leave the plants alone when they are small and susceptible to such things. I would suggest a gentle flushing with some tap water and h2o2 to oxidize and bad things hanging on the roots.
Plants in mediums generally like to get wet, then dry out, then get wet. Too much water isn't the problem, it's too little oxygen. Soaked rockwool = too little oxygen.