Cardinal tetra head down in the evening - spontaneously resolves??

[Pureguava]

Hi

I have a 15 gallon tank with 8 cardinals. Planted with CO2 injection. Plants growing really well.

Fish very healthy looking and feed voraciously. Getting tetra min and small mosquito larvae that are specially cultured and vigorously cleaned.

One and only one of the larger cardinals gets very head down and fights to keep down with the rest in the evenings. By the morning it has resolved. Does not happen every evening. First happened two weeks ago.

Any wise thoughts?

Ph 6
Gh and dh 0
Nitrates nitrites ammonia 0
Temp 26c

 

[fort384]

How long has the tank been established?

Is there anything else abnormal looking with the cardinal?

I will say I have had a cardinal exhibit this type of behavior before as well. He lived for almost a year, and the problem persisted almost daily the entire time, but he did eventually die.

 

[Jobby75]

My Neons do the this during the night period in the tank. Resting? Maybe he's just sleepy....