How long has the matrix been in your system? I run siporax, which is the same idea, although it's in my sump and not through a reactor. It's taken about 3 month's time to start showing the effects of it.
When you did your 40% water change, what was the effect on the nitrates? Did they drop and then climb again? Or did it just cut it in half?
The Matrix has been in the tank just shy of two months. Three months is what I had heard as well, to start seeing positive effects. Didn't think the Matrix would be pulling NO3 down yet, but was concerned that it might be bumping it up?
Regarding the water changes, the tough part is that the Salifert test kit goes from 50 at one color, up to 100 at the next color, so even after the change the color was basically still somewhere between 50 and 100...might have been 90 before, and 60 after?
Anyway, after doing some more reading, I think that I will start dosing some vinegar, as was suggested earlier in this thread. I have an apex DOS head that isn't being used, and will plan to use this to slowly dose throughout the day. Also plan to get some Brightwell MB7 to dose alongside the vinegar to hopefully prevent any cyano issues. Considered doing Aquaforest, or Zeovit, but neither system seems compatible with Kalkwasser - so vinegar it is.