I have 2 serpae tetras in a tank with several other fish. All the other fish interact with each other. My serpae tetras only hover in mid water and sit still in about a 45 degree angle facing downward. I have also noticed that they are getting kinda transparent. At feeding time, they will come up and eat with the other fish but once feeding is over, they will go back to hovering in mid water. Are they an anti-social fish?