Properly fill-out the troubleshooting form, and we can get a better idea of your situation. But with the info provided, looks to me like either nutrient burn or micronutrient burn.

You mentioned not using additives...does this include foliar sprays? How about Superthrive or one of the many bulking agents? Perhaps an iron or magnesium suppliment? Looks like repeated overdosing, but would need more info to confirm or dispell. My guess is magnesium or Superthrive OD. (just a shot-in-the-dark guess)

Then again...I'm not a hydro guy, and it might be something as simple as reservoir temps or nozzle clog. :wtf: