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Just for kicks, post ammonia/nitrite readings. Let's make sure this one fish isn't your canary in the mine. When I was cycling my 20 long for the first time, only 1 white cloud female was acting strange. All others were fine. I found nitrites rising at that time.

Just like humans, some individuals within a species can be more sensitive.

Other thoughts are - pregnancy? I don't keep female livebearers, so I don't know if the symptoms you describe could be that she is in labor. Do you see a spot in her belly (called a gravid spot). If so, she could be ready to drop her fry.

Look closely at her gills - are they moving faster than the other fish in the tank, especially when she is head's up? Is she head's up near the surface? This can indicate gill flukes.

If you see this and she is eating at all, buy the new Jungle Anti-Parasite Medicated Food, which contains PraziQuantel. It is effective against gill flukes. Others in the tank should eat it too. If she is not eating, you will need some other med required for gill flukes like Parasite Guard or Parasite Clear. Just make sure it states that it treats Gill Flukes or Flukes.

Follow instructions carefully and note if you have scaleless fish you may need to use at half strength and should remove inverts like snails.

What size tank was she in originally ? My immediate thought is that she may have a swim bladder problem and moving her to a smaller size tank has enabled her to regain her equilibrium.

ammonia "almost zero"

have you tested the ammonia in the 10 gallon that's housing the sick tetras?

my guess: ammonia/nitrite poisoning was making her sick, you moved her into a tank without it and as far as she's concerned the problem is solved.

I suggest getting test kits for nitrIte and nitrAte. In the mean time do a partial water change on your tank to make sure none of the other fish will be affected by the ammonia in there. Ammonia is considered toxic at as little as 0.1 ppm.

I just wanted to say that I agree with Tryst. Don't take "almost 0" lightly. Symptoms you described sound like potential toxicity.

After the water change suggested, add something like Bio-Spira, New and Improved Cycle, or Stress Zyme. This will help through the mini-cycle.