Fish dying not all at once

[Alien Anna]

I agree with everyone - I suspect your tank is not yet cycled properly. A nitrite level of zero doesn't tell you what it was a couple of hours before and the effects of nitrite poisoning can be long-lasting. I'm suspicious that your test kit may be inaccurate - take a sample to your LFS and get them to double-check. Don't forget to ask about ammonia.

The link in my sig would probably be useful background for you.