I started a 20 gallon freshwater tank last week. I filled it and waited a few days before buying any fish. The temperature has been very consistent at about 75/76 degrees, and the tank has neutral pH.
I bought a male and two female Platys 6 days ago, and they have been doing great until today. One of the females has become very inactive and is spending a lot of time in a certain corner of the tank (but does move top to bottom in that corner). This corner happens to be near the heater, but I have thermometers on both sides of the tank and it's a constant temperature. I've noticed that she moves her pectoral fins in a constant, slow, rhythmic motion. She also looks to be opening and closing her mouth, not constantly but it's noticeable. The other two fish spend time all over the tank and seem to be more lively in general. If they get too close to the sick fish's corner, the sick one runs them off.
I tested the nitrites, nitrates, pH, and alkalinity today, and they are all right where they should be.
Any advice would be greatly appreciated.
Edit: Her anal fin is not staying fanned, either. It looks more like the male's than the other female's.