Leopard Danio

[Dutsey]

Woke up this morning to see a danio hanging by the filter. It didn't look well. Appeared to have a redish tail. It did look quite big

Come home from lunch and the danio was dead

Has anybody had this experience before?

My Water results are

Ammonia 0
Nitrite 0
Nitrate 40

Please help

 

[leviethan]

hi, what is your ph, also please what is size of tank and other occupants? Thanks