Hopefully just a minor deficiency/toxicity but unfortunately it could also be spider mites(which is less likely due to you being in hydro, but still good possibility). Do you also have any soil grows nearby? I just had plants this summer that had very similar looking spotted leaves and I thought it was a deficiency but then ended up finding out it was spider mites when they webbed over a cola. Check underneath your leaves and look for spider mites. Do some research on em to know what to look for
If its not spider mites, then prolly nute problem. Whats ur PH and PPM/EC readings?
Best of luck man, hope its not spider mites, fuck those little fuckers!!!!