Celebes Rainbowfish dying
I bought 6 Celebes Rainbowfish not too long ago from my normal dealer. He rarely has die-off when ordering and keeping fish and has been doing so for 46 years at his current location. Everything I know about keeping fish, I learned from him.
I purchased 6 Celebes Rainbowfish from him recently, and they seem to be dying, one at a time, once per day. It started with the larger male, then the bloated female, then the smaller male, and now one of the smaller females. I have 2 small females left. In each case, all of the fish who died were not showing any signs of imminent death. They were all eating heartily, swimming and schooling like they should, all had bright coloration, and all had full fins with no decay present. My tanks parameters are perfect, 6.8 pH, Ammonia and Nitrites at 0, Nitrates at ~0, and a temperature of 72 f. The other fish, 1 German Blue Ram, male, 2 Lying Fox, and 1 Reticulated Hillstream Loach, are all doing very well and showing no signs of illness. This is baffling me. I just don't understand why these fish are dying, with one exception. The bloated female. When she passed, I inspected her belly, which looked "fuzzy" on the outside, and it looked like something ripped on the inside. In every other case, nothing like that has happened.
EDIT: I was able to observe how they die just now. One of my females swam to the surface and stayed there, about a millimeter from the waters surface, breathing fairly heavily. She stayed like this until she passed.