I would question the accuracy of using a marine test kit on freshwater. Even if the testing method is the same as a freshwater kit. I’m certain the color chart is adjusted for interferences present in saltwater.
Also it seems this is both a RODI storage and salt mixing bin. Unless you are cleaning the bin between each batch and bleaching it to oxidize any residual there is likely some contamination. However, this degree of cleaning would be an unnecessary step.
Regardless, if it is leaching nitrate it doesn’t appear to be impacting the tank that has zero nitrate. And the brown growth in the tank is likely dinoflagellates caused from the zero nitrate. This could be confirmed under a microscope.
Also it seems this is both a RODI storage and salt mixing bin. Unless you are cleaning the bin between each batch and bleaching it to oxidize any residual there is likely some contamination. However, this degree of cleaning would be an unnecessary step.
Regardless, if it is leaching nitrate it doesn’t appear to be impacting the tank that has zero nitrate. And the brown growth in the tank is likely dinoflagellates caused from the zero nitrate. This could be confirmed under a microscope.