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ALL FIVE of my new fish died OVER NIGHT. They were obviously healthy when I got them. They seemed healthy when I put them in the tank. Three were active, two were being stationary and hiding. The water had a pH of 8 and I honestly didn't test the water for ammonia. So that probably explains it. BUT, this was a new tank, which I introduced the dirty water to right before I put the fish in. The filter and heater were on for at least a week before, and the water should have been really clean. Maybe that little bit of food I put in for them was ignored and lay around rotting, and one of them died, and then that made the water filthier and they ALL died? They just all died, over night. It was unbelievable! The temperature was 78 or so, so that probably had nothing to do with it. This was very depressing and I have a feeling that I am doomed to kill these fish if I get them again. Strangely, my spiny eel (in a different tank) stopped fasting right after the sumbus died. Sooner or later I am going to find a way to keep some kind of compressiceps, but I don't want to kill any more of them! Maybe there should be a "how not to kill your fish" guide.

When all the fish die overnight in a tank,
I usually suspect a poisoning, or a heater malfunction.

You said your pH was 8, but was it only 8?
Some of the chemicals available for buffering,
such as Seachem Tanganyika buffer and Kent Cichlid buffer,
can be TOO strong when over-used.
I know of one case where a woman used a double dose of one of these, then did a pH reading with an electronic pen, and got a pH reading of 11. None of the fish died, but all were stressed.
She did a 75% water change, the pH went down to 9 and then all the fish were fine. I'd recommend using the Tetra pH test that reads
from 5-10 to get a better general idea of your pH.

Other poisons can be any of a hundred household chemicals.
Soap takes about 24 to poison fish. Windex, Raid, air freshener,
a cigarette butt floating in the tank, bitter apple, and probably
dozens of other items I can't think of can all kill fish in tiny ammounts.

I also suspect either chemical or metal poisoning, but another thing that can happen is a PH crash overnight can cause oxygen levels in the tank to plummet....your PH is 8 but is your water hard enough to support that PH? It happened to me once - luckily I got up early and saw the fish gasping...only lost half but it was shocking how sudden it seemed.

just guessing here though

yeh

it's also a possibility that the filter stopped running

or otherwise there might not have been enough water movement

I guess they had to be poisoned, because the temperature was fine. I noticed that the pH test I was sold (from ANOTHER STORE OF COURSE) was, surprise, actually for marine tanks. I don't know if this makes that much of a difference because that's also a high range of pH. The thing about all this is that I feel like other fish wouldn't have died, I don't know why. I set up a tank while my spiny eel was sitting in its bag and then put it into that tank, and the tank even had some cloudiness from the new dolomite, and it still did really, really well. But then again, these delicate looking compressiceps seemed to be doing really, really well at your store. If the sumbus come around again, I will buy some more and of course I will be extremely vigilant about the tank conditions.