I purchased 3 swordtails a weekago from a verygood lfs. The last one died today. They all swam around all the time and eat very wee with no symptoms of illness until they died. My water perameters are ammonia 0, nitrite 0, nitrate 10, water temp 77 degrees. Any idea what happened? All other fish are doing great. This is the second time this same thing has happened to me with swordtails.