It's not intestinal worms because they don't kill fish quickly, and the fish will keep eating while infested with worms.
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Breathing heavily near a filter outlet or at the surface can be poor water quality, something toxic in the water, gill flukes, gill fungus or gill bacteria.
Bacterial and fungal infections on the gills are rare and salt will deal with flukes, fungus and bacteria on the gills, so that's unlikely to be the issue.
You can test the water for water quality. Overdosing with medications could be an issue, as can heat and lack of surface turbulence.
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When fish stop feeding but show no external diseases, it's normally an internal problem. Internal bacterial or protozoan infections, or organ failure, or possibly a virus that affects their liver.
Internal bacterial or protozoan infections cause the fish to do stringy white poop. Organ failure will cause fish to swell up rapidly. If the fish dying are from the same batch and the other fish are ok, they could have a virus that is killing them. Or they might have been exposed to something toxic before you got them and their bodies are shutting down now.
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I have had similar problems with rainbowfish dying in soft water and it was caused by something in the tap water that was poisoning them. When I increased the GH, the problem stopped. I assume the minerals in the harder water neutralised the toxins in the tap water.
The only other option would be to take some of the fish to a fish vet and have them necropsy the fish and do tests to find out what is happening inside the fish. This will cost a bit of money and is not normally worth it. The Department of Agriculture sometimes has a fish health section and they might do the tests for free (for home hobbyests), but you would have to contact them first.