Slime/film in Flood and Drain reservoir

I'm experiencing a problem where after a few days some film forms in the bottom of my reservoir.
Both containers are completely light proof and currently I am not using an air pump in my res.

I am flooding hydroton just twice a day at the moment.
Using Maxibloom, Botanicare CalMag, and Protekt Silicon with Reverse osmosis water at 1k ppm.

The slime does not dissolve with mixing.

Run a sterile system by adding chlorine at 2ppm..