I had 13 seedlings and was misting them every day. They started to yellow and I tried giving them 1/8 strength nutes. They responded and started looking better overnight. I decided to use the nute again at 1/4 strength, they were about 3 weeks. The next morning the bottem leaves were covered in small yellow spots, they grew bigger and then turned brown like yours. I'm not there with you but I would say that looks identical to what mine looked like.... After a few more leaves formed I cut off the damaged leaves and haven't looked back. I haven't used nutes before a month since and haven't had the prob again. I know in hydro you have to put something in the water because there is no soil but you said you were already up to 900ppm, you didn't give their age but they look to young for that concentration....... Good luck, keep us posted.....