I would tend to go with the circulation/oxygen theory for a couple of reasons:
1. The rock and sand came from an established tank and was transported wet. There was an almost undetectable spike in the first week and since then all levels of ammonia, nitrate, nitrite, phosphate have been zero when i've tested the water. So no cycle to speak of.
2. The increased bioload did not seem to have any effect as I tested the water right after the fish died and all readings were still zero. Even if the fish were pooping up a storm they probably wouldn't die so quickly and suddenly as they did from increased waste in the water.
3. All the inverts and corals are doing fine. Good polyp extension on the leathers and the zoas are all open. Increased waste should have affected them as well.
So I think what I need to do is get my refugium going (have just been waiting for my led light to show up) and grow cheato in it with either a reverse day/night cycle or just light it 24/7. That will provide a constant source of oxygen to the tank. I will also try to rearrange the powerhead to get more surface agitation and exchange from top to bottom.
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