Is there any light in your sump? They are a type of algae which feed mostly from silica if I remember correctly and from my personal experience they don't show in a cycle until light it added.
There is a light in my sump. I also set up a QT tank back in February and I never once got Diatoms in there either. Is it possible that there were silicates in the trash can I bought and used to store the RO?


