Michael,
First of all, you make me check the nitrate level in our 120g before I stated that we don't have nitrate in our reef
0 ppm nitrate in there, btw
Second, I have doubts that you have any nitrate in your reef and suspect your test kit may be wrong. You have a well-lit refugium and lots of macroalgae to suck up nitrate, you feed sparingly, and you have a good skimmer (I presume as I know squat about skimmers).
I don't know how often you are removing the crud from your tank or if you are getting all of it - which may be part of the nitrate problem, if indeed you have one. We baste under our rockwork each week to get the hidden crud out into the open so we can siphon it out. This weekly habit may contribute to our 0 ppm nitrate, but I also think that the loads of macroalgae in the reef suck up most of our nitrate.
Anyway, with declining corals, I would first suspect that chemistry in your reef may not be taken care of on a regular basis. You did not mention alk, Ca or Mg levels, or how often you test and adjust these levels.
There may also be a problem with the chemistry of the pail of salt you're using, which must also be checked to eliminate this as a possible source of chemistry problems.
In our reef, I used to test and adjust alk, Ca and Mg every 1-3 weeks. However, our halimeda had always grown poorly, even though we dosed kalk nightly. Corals and coralline algae were okay, but not growing much. pH also was kind of whacky, sometimes being as far out as 7.98 in when the lights go on and climbing to 8.45 by lights out.
A few weeks ago, I went to dosing buffer, Ca and Mg daily, though I only test every 1-2 weeks. I have already noticed a positive difference in the growth of coralline and corals. pH has also stabilized between 8.10 and about 8.35.
Actions I would take if your reef was mine:
- double check nitrate levels using a different test kit
- test current pail of salt for chemistry problems
- set up a weekly schedule for chemistry testing and adjusting and keep a notebook of what your results are and what/how much you dose
- set up a weekly schedule for siphoning out all the crud, including the stuff that's under your rockwork
HTH and let us know what you find