Nitrate may be the issue, it may not be. This is biology, not chemistry or math.
If you do add nitrate, you don't have to add much to see a difference. Be careful so you don't end up with 10 ppm. Ha! Aim for 0.25 ppm, and add that dose once every couple days until you achieve 1-2 ppm or whatever you decide you want to stick with. If this
is the issue, then you should see a difference within 2-4 weeks of achieving a readable nitrate level. For many tanks, this is all the difference in the world. If your tank is using close to an equal amount of nitrate that is being supplied already, then you won't see a difference.
The other thing that comes to mind I've already mentioned.
Chemical warfare. Do you have any other systems that you could experiment by removing some softies?
Another is rock steady parameters. Aside from testing imperfections of both the user and the accuracy of the test kits, you parameters need to be steady. Steady is more important than the actual number. Such as, whether the alkalinity is 7 or 11 dKH, it needs to remain within 0.25 dKH each direction, all the time, on every single test, for many weeks/months running.
Steady parameters also includes when you do water changes. For example, if you like to keep 7.5 dKH in your tank, don't use Instant Ocean (for example) because the bucket mixes to about 11 dKH, and every time you do a water change, especially if you do a bigger water change, you're going to get a small spike. Make sure you you match salinity, temperature, calcium, alkalinity, magnesium (the "big three) of incoming new SW. These things matter. Watch out for an upward creep of salinity if you're dosing lots. Check the tank salinity before every water change, and adjust when needed. The upward creepy of salinity makes all parameters more concentrated.
A note though, that I've found, is that you achieve the best results from rock steady parameters, but I've also found that these tanks are the most sensitive to changes when there are changes - such as power failure or heater failure or something like that. These tanks are used to no changes, and really dislike changes. So sometimes a person may not want to achieve the best of the best if that means the system may be more finicky.
Something that I've done for almost two decades that I've not really come across anyone else doing (for the same purpose) is, 3-4 times per year I do a very large water change. In the neighborhood of 50-75%. Polyp extension for the weeks afterwards is significantly increased, and everything seems like it gets a perk up. I make sure every parameter I can measure for is matched. Years ago I never really understood why these big water changes make such a difference in my tanks, but now I wonder if it is helping to flush chloride from the system so chloride can be replaced by more important trace elements that we don't test for, but also contribute to salinity/specific gravity. Check articles for information on excess chloride and ionic (im)balance. It's widely discussed, not widely agreed upon. It's just something else for you to consider.
