I have no experience with this product but I did read up on it as I am not familiar with it and I was curious...
Bottom line - fish tanks require water changes on a regular basis due to the trace elements that are in the salt mix - our fish and (mainly) corals require these. If you are not doing water changes, they are not getting these elements.
I read this is also mentioned on the website for this product - that you may not have to do as many water changes - but you will start having to dose all of the missing trace elements, which means you WILL have to test for them all as well. Daunting task imo.
Keep in mind you will also have to do water changes to keep your phosphates down anyway!
The other thing that stood out was this seems to be just another way to carbon dose a tank - as you DO have to inject this system with sugar/vodka weekly and make adjustments to the unit every few days (per their instructions.).
It might be easier to just build your your denitrifier (do a search here) out of a few PVC pieces and a ton of airline tubing & a few little valves.
If you already have a nitrate issue, you need to find the cause of it and correct that so it is not an on-going issue.
Better yet, practice proper husbandry by feeding intelligently, doing frequent water changes, maintaining a sensible bio-load and running carbon & GFO reactors
