Im calling BS on that.
For starters, if its a biological media, there is no way it can be used for both salt and fresh, the bacteria are too different and cannot live the alternate conditions.
Secondly, you cannot have any oxygen present for the last stage of converting the nitrates to nitrogen gas, that is why some people have a deep sand bed, if you have water just flowing through there, there will be oxygen present and you arent going to be converting nitrates.
Thirdly (?), you should not have to feed the media, it should be feed from the stuff in the water going through it. If you are feeding it sugar water, which is providing them with glucose which is the best food source, they have no reason to work on breaking down the stuff in the water column.