I seem to have a problem with my swordtails. Everytime I get some they always seem to develop fugus on their tails and eventually die. I tried using some fugus cure but it ended up killing all my swordtails. None of my other fish are affected by this fungus and I did use an accurate dose. Whats wrong? I managed to keep 1 for quite a while but it just died without any signs of injury or disease. Just stopped moving after if was resting in a plant??? Any suggestions?