A. Australe with breathing issues.

While it's far from a conclusive diagnosis once water quality issues are ruled out breathing trouble is usually parasitic in origin, if you see no visible signs it's probably flukes. Praziquantel is the drug of choice. Treat the whole tank, prazi won't hurt inverts, plants , or your cycle.

Hmmm - what's "safe levels" for nitrite - the only safe number is 0. Anything else can lead to problems with oxygen uptake - exactly what you're seeing.

Your ammonia is at .25 . That could very well be the cause. Ammonia irritates the gills of fish and will burn themm eventually, causing death. You have fairly low levels but you should try to correct it immediately, as you don't want it getting worse.

An ammonia level of 0.25 is only harmful if the pH is relatively high. Ammonia readings without pH readings are meaningless. If the pH were 6.0, ammonia could be 5ppm or more and the fish would be unharmed.

Can we have some numbers, Eupterus? "Safe" doesn't mean anything - is it actually 0? If you're using paper strips I'd get a proper liquid test kit because the paper strips are notoriously poor.

What does "high side" mean for pH? If it's above 8.0, then some fish (they vary in susceptibility between species and between individuals) at an ammonia lovel of 0.25ppm could have issues.

I agree with Karl that ammonia (NH3) result w/o pH results does not tells us much as far what needs to be done to correct water conditions. Would not do any unnecessary pwc, especially massive using straight tap reagrdless of use of dechlorinator, without knowing what is taking place w/i tank water to our best ability.

As far as your comments about why just one fish.
Ive kept Hundreds of one type of fish in one tank. In case where water started to deteriorate gradually, it seems that not all of them will begin to show symptoms of stress/complications. It always start with few and gradually will spread to others with same /similar symptom unless water conditin is improved. I believe that each individual fish has somewhat different thresholds. I also have seen whole tank gets wiped out ovenight. In such case, it is my understanding that changes w/i water probably have been abrupt, not gradual.

Therefore, do not assume water is OK because only one fish shows symptom(s).
Perform test regularly before deciding action(s).

By comparing few test results (yest, today, tomm), you should be able to determine if your tank water is in the direction of deterioration or improvement.

It is also possible that it is beginning stage of infection. But let's rule out the water issue by monitoring before tempting medications. Who knows it may have came with such to begin with which no one can be responsible since NO ONE can be 100% accurate.

Clamped tails and fins are not usually good sign though. Just make sure that clamped tail/fins does not progress to bactierial/viral infection.

Although A. australe is OK in unheated tank, constant fluctuation of temp may not be good idea in tank which may have housed different fiish previously.
I have kept many different Sp of Killies in jars w/o mishap but my fish room was always kept pretty much in contant temp.

Another issue, unlikely, is dissolved oxygen level especially in the morning in well/heavily panted tank. Check the pH difference in the morning before the light and one before the light goes off in the evening..

Lets rule out the ammonia, nitrite and pH issue. Post the some numbers if possible.

Any pics?

Hope all goes well.