I have 2 female guppies in a ~60 L planted aquarium, temperature ~25 °C, internal filter, relatively hard (but not too hard) water. They've been on a diet of standard flakes. Change about 1/3 water every 1-2 weeks. Until recently, they shared the aquarium with a male; all adults. There were another 3 females and another male prior to that. The females were placed into the aquarium about 2 months ago, the males maybe a month after that. One by one fish have been dying. The first to die was a female which looked to have a small white blotch, something I did not notice at the store. Another two females and one of the males died shortly after after losing weight, not always showing clear signs of the culprit disease (I assume water conditions and food were ok). I don't know the age of the fish, but the females are/were pregnant when purchased.
Despite his small size, the surviving male had been harassing the females, following them and biting them at every opportunity. After the latest casualty of unknown cause I concluded that the stress of being chased may have been a contributing factor to the deterioration of the other fishes health, so I decided to remove the male to another container. Because some of the fish clearly suffered of some disease I presumably cannot blame the male entirely. In addition, I noticed that a fry had been born, and wished to minimize the chance of cannibalism.
What has me a bit puzzled is that now that the females are not engaged in flight behavior, as they are not being chased, they seem to forage more, and I fear that they might instead now cannibalize the fry. This has left me wondering what strategy the male was pursuing in chasing the females? Might the harassment actually serve to protect the fry? I doubt that this is the males progeny as the females were already pregnant, so I am assuming the male was simply attempting to destroy the pregnant females together with any eggs or fry within, but I am not entirely sure.