Hasn't happened yet but my 1 eyed dwarf neon rainbowfish is at the top of the tank breathing heavy. Partial weekly water change was today, water was/is spot on. He still eats really well, tank is oxygenated (added a little more surface tension just in case) and the only real difference is the temp was changed 2 degrees and I added a few nerites.
55 gallon, planted tank, 78 degrees. Have some nitrates but under 20ppm. He is 1 of 8 rainbows.