Blue Acara, problems staying upright

[Kitiara]

My female blue acara seems to be listing to the right and having problems staying upright - sometimes she is completely on her side. I have just put her in a small isolation tank where she is pretty much staying still (but looks to be having balance problems) In the larger tank before I isolated her she would stay in place on her right side, pick up and dump a few mouthfuls of gravel then move off to another location and repeat. When swimming she does not seem to use her tail much and gradually leans more and more to the right. She has no obvious sign of injury and her markings have changed - instead of having vertical black stripes she now has one horizontal black stripe)

All the other fish in the tank (larger male blue acara, 2 salmontail catfish, a silver shark and a spanner barb) are all normal. Their diet has not changed. She is about 6 inches long and part of a breeding pair I'd hate to lose her! She is currently on her own in a 1 foot isolation tank so the other larger fish won't pick on her (the shark and catfish are twice her size)

What is her problem and what can I do to help her?

 

[Liam]

is her stomach bloated?

 

[Kitiara]

no, she appears normal other than her swimming and her horizontal stripe instead of the multiple vertical stripes

 

[Kitiara]

My girl is still hanging in there. From searching the net I suspect it may be a swim bladder problem. I'm currently not feeding her (offered her some pea but she wasn't interested) and have put some melafix and pimafix in the tank just in case. She is spending more time on her side now - looks almost paralysed when she does it and sometimes stays that way for a couple of minutes before managing to get herself upright again. Now she lists to either side and is occasionally nose down for brief periods.

Her partner still in the big tank is starting to fret now too, he's obviously looking for her :-(

 

[Kitiara]

Took some tank water to the local pet shop for a test and found nitrogen levels too high. Have given the tank a major clean out and replaced 50% of the water. In the process I found that the external cannister filter had stopped pumping for some reason. Unknown how long it hadn't been running for as the thing is practically silent - the air pumps drown out any noise it makes. Filter restarted when I unplugged it and plugged it back in so I cleaned it out too. will keep an eye on it to make sure it doesn't stop again.

I believe this to be the source of the problem - no filtration for who knows how long. The acara is still with us, but she is not showing any signs of improvement