Mysterious Death

[Xenity7]

I have a 20 gallon freshwater tank with Eclipse hood and a 25 watt heater. I have 5 neon tetras, four lamp-eye tetras, and 1 Zebra Danio.
I used to have 6 Danios but five of them died mysteriously, they were all eating, all tank parameters were normal, and suddenly in the space of about 3 days all but 1 were dead. The Lamp-eyes and Neons are fine. What would kill 5 of my danios but not the last?

Ideas, anyone? :l

 

[smb]

What were your ammonia,nitrites and nitrates when you checked them? How long have you had the tank set up and the fish in there?

 

[golfboy444]

check your water parameters. sama: :saddam:

 

[Feline]

How long ur tank's set up? N how long the danios have come home?

 

[Xenity7]

The Tank has been setup for about three months, all parameters are zero, and the Danios are what I used to cycle the tank.