I have a 20 gallon tank with some Swordtail fsih in it 4 adults and a few babies. I have been loosing a few fish off and on. I have many years of experience with tropical fish and I can't figure out what is wrong. The babies seem fine it's the adults. They all seem to have their fins down and they just stay in a corner and wiggle in place. I have tried a fungus medication and that doesn't seem to help. I have also used salt. My PH is 7.0 the water is hard and I hane no nitrates. The temperature is kept at 79.0 and I do a water change once a week. I did a 50% water change yesterday because they didn't look well. I have no ick, no fungus that I can see, no tail rot. Can any one help me?