New fish dead out of nowhere

[TheDudeAbides]

My tank has been established since June/July, and water parameters check out ok regularly. I added some new fish a little over a week ago, and found a Black Phantom Tetra dead yesterday. I had a total of 12 fish in the tank… 11 Tetras of a few different types, and 1 Pleco.
Not sure if this was due to any kind of shock, but the fish seemed fine until it wasn’t. No signs of illness or stress. I’ve had this problem a few times before and cannot figure it out, only there’s usually some kind of early warning signs of something not right. This time, it was fine one day and dead the next. What am I doing wrong? I have two different types test strips and I never seem to find any issue with the water balances.

 

[Aiken Drum]

What precisely are your water parameters?

How long has the aquarium been set up and running?

Tetras are social fish and do better in groups of their own species. 4 is better than 3, 6 is better than 4, 10 is better than 6 etc. You really should be looking for a minimum of 6 tetras of any species. If they are kept in too small groups they get stressed, might get aggressive, not eat, might just get on with things, etc.

How many black phantoms are in your aquarium?

Test strips are notoriously not accurate, so your water parameters may be off even though the strips say they are fine. But let's work with what we have for now.

You also need to factor in that not every fish you buy is going to be healthy.

 

[Andy Sager]

Adding to Aiken's points, what size ( gallons and dimensions) is the tank? Was the dead fish a male or female? Was the dead fish a new arrival or an older established one?