I have a 5.5 gallon cycled tank with three platy sisters. For the past few weeks I've noticed that one will go into these bouts where she just stay at the top in the tank in the corner with her gills rapidly flaring. I know there's enough oxygen intake and the others act totally fine so I don't think that's that problem. She will do this for quite a while and then occasionally stop to swim and forage and act normal and then goes back a few minutes later. These bouts usually last for a day or so. There in no ammonia or nitrites but the nitrates have been a bit high(80 ppm) and I'm trying to lower them. But if that's what was causing it wouldn't the others act different as well? I do a 20% water change weekly and the temp stays around 78 degrees F. I've also noticed they have had long strings of(usually white) poo but I feel if it were parasites she would show more symptoms by now. Help!