I think we can exclude that such a concentration of Se can make it through your rodi. Please take a look at the Wikipedia article about Se: The Tolerable Upper Intake Level for humans is 400 µg per day. 2 liters (0.52 gall.) of your tank water would exceed this level. I think this concentration would not be tolerable for tab water and you can estimate that maybe 2 to 5 % of the ions in your tap water make it through your rodi.