i assume ferts arent an issue since ur dosing NPK+Traces? IF so, its CO2. Also, i have noticed that some plants get hit by diatoms worse than others. On my hydrocytle, the diatoms cannot be wiped off and the leaves start dying. That led to me think its something else, but on my rotala it can easily be wiped off. I know my issue is CO2 since its definitely not ferts.