bad news for me

bad news
I lost my 3 ornatipinnis, I do not understand the 3 at the same time there are three days I have to eat, there was one who was scratching in the sand but no apparent injuries.
he was in a large tank 120x40x40 he was 8 cm, temperament shy but good to go
PH6.8
Conduc350
we say poisoning, like that of the 3 at the same time it's weird.
as there are in the cellar and the aquarium was not fully covered is it possible that it likely comes from a fungus or saltpeter?
I hate
I've never had any problems with my wild bettas and there with channa, 2 orientalis death ornatipinnis now.
what should I look for it in the water?
it's still a dramatic death.
eric
sorry to write in English even if I go through a translator I know what I say but not in German: (
eric