I have a 33 gallon/ 128 liter tank with 5 Swordtails, 3 Platies (plus one fry), 4 Otos, 2 Zebra Danios, and live plants. I quaranteed all fish for 2 weeks before adding, and the water parameters have always been fine (ammonia, nitirites, and nitrates 0, pH a stable 8.0). A few weeks ago I...