What fertilizer are you using?
. I would have my ppm at like 400ppm when they were small then working up from there to 1000ppm as they got bigger into production.
I would just keep ph at 5.9 and water temps good and I never really had problems with hydroponics. I am not saying switch to soilless drain to waste, but that is what I went to after trying DWC and flood and drain, and what I found easiest for me.
Sorry I can not say for sure what the problem is, I never really seen that before.