Thanks again. To answer the pending questions...
I feed my fish tetramin flakes with some crushed crab. They've all eaten it, but of course, they only eat for a couple of days and when they start getting stressed they'll stop eating. When vaccuming, I normally just touch the surface of the sand to get any loose matter. The last time I did it, I did a deep vacuuming and a 50% water change just out of desperation. My only remaining fish (foxface) is still alive but its breathing is labored and doesn't look too good. It, however, has survived longer than any of the prior fish as it is going on 9 days now.
My protein skimmer seems to get very little dark water in it. Maybe 1/4 inch in the cup in a week and it is set optimally per the instructions. It's one of the good ones that creates a vortex in the chamber and you adjust with the little bleed valve. I monitor ammonia/nitrites on a daily basis and have never seen a spike in either one. I use the master test kit and I also have one of the little ammonia detectors hanging in the tank but I don't depend on just one test so I keep testing with the master test kit just to be sure and I never see a problem. I use a professional bulb tester to test salinity. I don't add any additives when changing water as I use the RO water. When I was using tap water, I only added a dechlorinator. Using tap versus RO water doesn't seem to make any difference, but I've used exclusively RO water for the past month.
I also never change/rinse both filters at the same time, but I don't think that's an issue anyway because I never see an ammonia/nitrite spike. I think the theory of an unknown toxin is the best one, but I'm at a loss as to what to do about that or where to look. With the 40 lb of live sand and ~10 pounds of live rock, I didn't think I'd need to add any more of that. Plus, again, with no ammonia spikes, that should mean that I have enough LR/LS for the bioload. No?
Oh, and I have the lights on 15 hours each day, just because that's the time that the room is used by people. Should I cut back?
Mike