I have a few questions/comments for you that may help everyone figure out the problems you've been having with your tank.
1) Did I read correctly that you first added water to your tank and started your cycle about the beginning of Feb? If so, I believe you may be moving to quickly.
2) Your tank is at your office, is the heat/ac left on in the building even on the weekends? Does your office temperature fluctuate with your door open or closed? If the heat/ac is not left running all the time in the building you could be having temperature swings.
Hope it helps some.
I did fill the tank and cycle it at the beginning of february. Before that time I had been curing 25lbs of live rock in a plastic bin at my house for a good month, then moved the live rock to the display tank with around 125+lbs of dead rock. The stuff on the dead rock started the cycle, but it was admittedly a small cycle (I posted here to get opinions on whether to add a shrimp or not, and general consensus was that it the tank was already cycling that it wasn't needed). The tank cycled for a little over 2 weeks before I added my first fish, which were 3 small chromis. Again, I know it is not common to add multiple fish at once, but since these were schooling fish the consensus here again was that it would be okay to do this as my first additions. I intentionally chose small fish so as not to add a huge bioload to the tank (2 were about 1in, the other is aorund 3/4in in size). I bought all three from live aquaria, along with a small cuc of 15 mixed snails (banded trochus, margarita, nassarius, cerith, nerite, astraea conehead), and 2 corals. Since adding the fish my parameters have been consistently good (ammonia 0, trites 0, trates 5-10ppm). The first fish that I lost arrived with a small protrusion on his head that got progressively worse. He was bullied by the other similar sized chromis and I believe stress led him to be unable to recover. I also believe he came to me sick and that it was not something I did. Since all three chromis were shipped in the same bag, it is possible that bullying went on during shipping. It is unclear to me why the other one is now sick, other than to say that the black spot is similar is size and appearance to the first fish, just in a different spot. I have been testing daily and sincerely don't think it is something that I have done or something in my tank - all of my inverts are doing well, as are my corals (I was concerned about one of them earlier this week but think I was over-reacting, it is looking great).
The temperature question is a good one, but I don't think this is a contributing factor. My office building is pretty consistent - no air conditioning running this time of year and I have individual control of the heat in my office. I've been here late at night and early in the morning, and temp is the same as mid-day. I have a JBJ heater with external controller and probe - over the month and a half that it has been in my tank (and the month that it was in the bucket of curing live rock before that), I've watched it keep my temperature very steady within less than a degree of change over all that time. I also have LED lights, so they do not contribute much if any heat to my tank throughout the day.
I haven't set up a grounding probe yet (but I do have one) so that is something that I could do, but I would think that if there was something going on with stray voltage or poor water quality that I would see problems iwht my inverts before I would with my fish, and all of my inverts are quite happy, haven't lost any or seen problems with any of them.
I've been doing small water changes (5-10g 2-3 times a week) to keep nutrients and trates in check since I've been feeding more often with the illness - is it possible I'm doing too many water changes? I don't have a skimmer up and running yet (there is a thread awhile back on my sump problems if you're wondering why!), but many people just use water changes and live rock as their filter without a sump, so I haven't been terribly concerned about that. I've had the normal algae and diatom outbreaks, but nothing too severe or that my cuc hasn't been able to handle (I've since added more snails to help out, and they are doing a good job).
I wish there was something really clear that I was doing wrong, but from what I can tell everything except my fish are doing great in my tank. It is hard to know what else to conclude other than they came to me with something already going on - and I know for sure one did since he already had signs of it when he was in the shipping bag. Do you have any other ideas?
North - that's a good idea to submit it to wet web media - maybe I'll go ahead and do so just so that I have more information.