Twitchy cardinal tetra

[Scott_R]

I have one tetra that is floating in the same spot near the gravel and twitching. I don't see any spots on him.

Ammonia and nitrites are 0; nitrates are 5.

Here he is:

All of the other fish (cardinal tetras only) are together in a school.

 

[Delapool]

Hi, kind of hard to make out for me. I'd try a water change. Any chemicals, new wood, plant ferts or a very big water change been done recently?