I agree with you. Sulfur based nitate reducers bascially replace nitrate with sulfate and, as a result, have little place in a freshwater tank. They are fine with saltwater, because saltwater is already high in sulfates. Freshwater is not and I would consider the impacts of the having sulfate levels that high in a freshwater tank are not much better than the nitrates. In addition, the outflow from a sulfur denitrator will be very acidic. In a saltwater tank you can (and must) push the outflow through crushed coral to increase its pH. Doing the same in a freshwater tank may cause the pH to increase too much and will increase overall hardness and TDS levels. Again, probably not an effect we are looking for with freshwater fish, at least not those that prefer soft water.
When it comes to reducing nitrate levels, salt water tanks have an advantage over freshwater tanks.
NASA is working on a denitrification filter for raising fish "from egg to egg" in space. Hopefully that technology will trickle down to us as well. It will have to be small (a word apparently not in your dictionary
), effective, easy to maintain, and will be designed to keep nitrates below 21ppm.
You are doing exactly what I have tried to do, with some success (not as much as I hoped for), on a much smaller scale. I am anxious to see if "scaling up" makes things easier and more effective.
