My fish died overnight mysteriously?

[laingdda]

I have found one of my tetras dead right in the corner of my fish tank and I am really unsure what caused it. I found it tight in the corner upside down dead. The water has been checked and is Ok. I did a 30% water change as normal the day before. I can’t see 2 of my other 4 tetras either. I only got them from the shop 4 months ago. All the other fish seam fine as well as my remaining tetras. I have 4 corydoras and 6 guppies with my tertras. Any advice would be appreciated on how my fish died and how to prevent anything similar.

 

[laingdda]

I have found one of my tetras dead right in the corner of my fish tank and I am really unsure what caused it. I found it tight in the corner upside down dead. The water has been checked and is Ok. I did a 30% water change as normal the day before. I can’t see 2 of my other 4 tetras either. I only got them from the shop 4 months ago. All the other fish seam fine as well as my remaining tetras. I have 4 corydoras and 6 guppies with my tertras. Any advice would be appreciated on how my fish died and how to prevent anything similar.

I also want to add that it was a bit smaller than the other tetras and the temperature was up 3 degrees

 

[Essjay]

Can you tell us what type of tetras you have, please.
Were there any signs of disease on the dead fish?
What size is the tank?
What were the levels of ammonia and nitrite when you had them checked?

 

[laingdda]

Can you tell us what type of tetras you have, please.
Were there any signs of disease on the dead fish?
What size is the tank?
What were the levels of ammonia and nitrite when you had them checked?

It was a rummy nose tetra and the tank it 85litres. No sign of disease. Ammonia is showing as 0 and same with Nitrite.

 

[laingdda]

Water test