I'm leaning toward toxicity/poisoning OR a sudden low oxygen issue since the smaller fish survived.
Cyanide capture can kill fish but it's not contagious.
Airborne toxins, or a power failure (no flow... low oxygen) would be my prime suspects.
I have seen hand lotion on a hand introduced to a tank, wipe it out too...
Unfortunate loss
Jenn
Edit:
Thanh386;705391 wrote: The fish you purchase, was it someone you trusted or just a deal. I dont think it a common practice anymore, but fish sometimes are caught with cyanide/toxin. The toxin doesn't always show up until its too late. If you accidentally or intentionally added the water that the fish was brought home in, and he was caught by less than ethically means, the water could have contaminated and killed you're fish.
But i dont know if you can test for this, I suggest running carbon massively right now.
Cyanide capture, unfortunately, is still commonly practiced in some parts of the world. There is no universally accepted test for this, and the testing that has been done in the past, involves sacrificing a fish and examining its tissue within 24 hours of capture. The fish literally pees away the evidence within 24 hours of exposure. Some fish survive it long-term, many do not (most?). It can take some weeks for them to die after the fact, usually after the fish has reached its destination (the consumer).
However, this does not "transmit" to other fish. So even if one winds up with a poisoned fish, it won't spread. I suppose if a specimen dies and fouls the water, that can kill others, but in OP's instance I don't think enough time passed to cause ammonia to spike, etc., and based on the photos of the bodies, they were removed promptly after they were discovered dead.
It's more likely to have been caused by one of the causes I cited in my previous post.
Jenn
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