Kuhli loach breathing heavily and hiding

[emmaorabelle]

Hi everyone,
I’m hoping someone can help with one of my kuhli loaches. I have a 75-gallon freshwater tank with six kuhli loaches, eight harlequin rasboras, and a bristlenose pleco. The tank has been running for about a year.
For the last three days, one of the kuhli loaches has been staying under the driftwood and barely coming out at feeding time. Yesterday I noticed it was breathing noticeably faster than the others and occasionally rubbing its side against the sand. I can’t see any obvious white spots, wounds, or fungus, and it still looks reasonably well-fed.
My water tests are ammonia 0, nitrite 0, nitrate around 15 ppm, pH 7.1, and temperature 76°F Basketball Bros. I did a 25% water change yesterday, but the breathing hasn’t really improved.
Could this be an early sign of gill irritation or parasites? Would you quarantine the loach at this point, or monitor it for another day or two?