Sudden Deaths

[munkey2k2]

Over the last 2 days ive had 2 ruby nose tetra and my siamese fighter die. Did a water change on saturday as normal. Tested the water and all levels are fine. What could be causing this? How can I stop my others dying?

 

[WILDER]

How many gallons or litres is the tank.
How many fish and which type.
What test kit are you using.

Any symtoms to go on.

Could any chemicals gotten into the water during the water change.

How long has the tank been set up?