Thats what I thought too. But I've never had a bacteria bloom kill a fish beforeSorry for the loss.
The water turned white? Bacterial bloom?
I used a filter and substrate from established tanks so I thought that'd be fine. Guess notI have never had Nitrates be zero in an established tank. The fact that you are reporting zero makes me think that the tank was cycled properly.
So I tested the water. Nitrates 0. Ammonia 0. Nitrites 0. Fish was fine. He was eating. Then overnight the water turned WHITE and he was dead