Hello!
If it was the food – they got (along with naupliae) new black mosquito larvaes from my balcony and microforms …
Both can came along with too much bacteria …. perhaps ….
The other fish, who get the same food, are o.k. …….
I think that the animal was sick and weakened or they will not give so quickly and are quite capable of resistance. Unless that “often” happens you need to make thee no great thoughts, even when this happens to me sometimes.