I have a tank that's about a month old. I lost two guppies today and it's a bit of a mystery to me. There were four zebra danios, three male guppies, an oops female and two oops fry. Tank parameters are good. 0ppm ammonia, 0ppm nitrates, 5-10 ppm nitrites, 76F. The tank is a 20 long with A HOB...