Planted Tank Guru
Join Date: Dec 2017
Location: Elk Grove, California
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.
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