Its not a parasite.
There are no parasites in aquarium that can infect and kill existing fish ( the fish you had in your aquarium prior to adding new fish) in 24 hours. And, you would see visual signs on at least one fish if it was Ich-- which is one of the quickest- if not the quickest external parasites to complete their life-cycle and kill fish.
Also, parasites dont kill fish "all at once." Even with Ich, fish will die at different times.
You could have had an issue with water quality. The large amount of introduced fish causing an ammonia spike because bio-load maxed out the bio-filter.
Columnaris- one of the only diseases in aquarium that can kill fish in 24 hours.
Thank you Amy. Whatever is affecting my gourami is 100% my fault.
The existing fish have not died, but the sparkling gourami is definitely not well. Fins are clamped and it seems to have difficulty swimming. They managed to eat, which I hope is a good sign.
I also have 4 corydoras habrosus in there but they do not seem to be affected by whatever it is that is that the gourami have.
Did a 50% water change yesterday when the gourami were still behaving normally and another 50% today after I noticed how lethargic they've become. If it's too fast to be a parasite, then probably something bacterial? If it is columnaris then I guess i'll be removing a bunch of dead fish from the tank tomorrow morning :C
Assuming they make it through the night, would General Cure be a good idea or should I try something else?