I have 7 Serpae tetra in QT and just noticed today (2 weeks in qt) that one is swimming head pointed down with a coughing motion...kinda like he is going to look for food in the gravel but isn't. Parameters are 0/0/20, just did a 50% pwc in the event it was related to the nitrates (my main tank never gets above 10). I don't see this behavior in any of the other tetras in the qt tank. Any insights or recommendations?