Ok so about 1 and a half weeks ago I got three guppies, one male two female, and my sister got one of each. Well in the past two days my sisters has died. While she was in my tank and my yougest female has also died. Both of which were pregnant. My male and oldest female is still alive as is rest of my fish. I woke up and both were behind breeder net. So what killed them? My tank is around 73 degrees and the ph is 7.4