what are we not testing for?

After reading about your issue, I have a couple of questions. First, I'll state that I think ammonia/nitrite/nitrate one or all 3, might have played a role in the ich.

You state that "There was no elevation at all in ammonia/nitrite/nitrate. However, my other fish were obviously stressed by something. .. "

Did you test for these 3 items immediately after the flame angel went missing? Or did you test after several days had passed? If yes to the former, perhaps none of the 3 had a chance to yet develop. If yes to the latter, perhaps your biological filtration was efficient enough to nitrify all traces of the 3 compounds by the time you thought to test.

As I'm sure you're aware, the ich-causing variables are ever present in our tanks. It only requires periods of succeptability of our fish.

Without knowing anything else about your setup, and by guessing simply from your assessment of things, it would seem that the death of that single fish prompted an ammonia spike, followed by nitrification, and eventual riddance of these compounds by your biological filtration.

During that period, several other fish got ich. I would think that at this point, the main damage is done, and moving them from the DT to quarantine might not accomplish what you're after. If it were me, I'd maybe leave things alone. Monitor the condition of the hippo and the others. If things get worse, move them to quarantine.

I'd probably just give it a day or two.

And to address the question of yours, "is there something else noxious from the decomposition of the fish that stressed my other fish. Maybe a sulfur compound? I just wonder if there is something else we should be testing for on a regular basis. . " I'd think not.

In my opinion, a fish died, and its remains went unaccounted for, during which time other sensitive fish were affected by an ammonia spike. If I'm near ballpark; said spike is over. Monitor your tank and keep us posted.

Good luck!

 

You may well be right, reefsparky. I did test the water each day since the fish went missing. However, it only stands to reason that I had some ammonia which was very quickly converted to nitrate, which was then very quickly gotten rid of either by my fuge, live rock, or DSB.

I had thought of leaving them alone for a few days, especially since they are still eating. If I tried that, I might consider purchasing the Marine-Max product that Unclejed recommended to boost immunity. However, my worry is that my hippo is absolutely covered in white spots, even on his eyes. So I thought the time may have come to intervene with some serious meds. And I thought that if I treat him, might as well treat all the fish and try to eradicate the ich from the tank.