Platy fries dying slowly. Help!

[Muchcha]

Hey! One of my platy laid around 35-40 fries about 2 and half weeks ago.. Now 2 has died and 1 was going to die again,then I took it out from the fries tank and put it in a small cut with some salt.. It worked.. Now the one which was going to die is swimming well.. But I want to know y that happened.. Does anyone know? I know I have 35+ fries but I don't want them to just die.. So if anyone knows anything.. Please share with me? thanks you!

 

[jlk]

What are the parameters in the fry tank? If one that was sick recovered once it was moved the new water, it makes me suspect that the water in the fry tank has some water quality issues happening. Fry tanks need to be kept in an ideal condition ( zero ammonia and nitrite with very low nitrate) because fry are extremely vulnerable to any level of toxins.