How long have you had the tetra?
Have you added any new fish recently?
How much water did you change on the tank, and did you use conditioner?
Have you checked your water parameters, especially ammonia, nitrite, nitrate, and pH? Do you know what the pH of your tank normally runs?
I bought it 2 days ago.
I have a beta fish in a big bowl and decided to get two more fish as there is heaps of space.
I went to the aquarium and the guy there said tetras will be okay with betas.
He said Black Phantoms and Rummy Nose tetras would be fine.
I have a heater for the water but not a filter and I am not sure about how to check PH level etc.
I have just discovered these tetras should be in groups.. I am kind of annoyed that the guy sold me school fish by themselves.
As soon as I have enough $$ I am going to go buy a filter.
Also he knew I didn't have a filter and I am starting to think I need one for these fish.
EDIT: I changed half of the water.
Also, the Rummy didn't survive the night.
The Beta and Black Phantom seem okay though.