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Background

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.

Question

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.

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Guppy males harass females quite aggressively, and it is therefore recommended that you have multiple females per male. Males will also flaunt each other and harass each other. Males exhibit this behavior, because reproduction in guppies works like this:

  • Males tire females until they are slow and cannot avoid them.
  • Males then make swooping passes with their genitalia to basically stab the females with sperm.

I believe why this evolved is to ensure the strongest males are the ones which pass their genes on.

To ensure the fry survive, you will need areas which they can hide easily, such as in high density plants where adults cannot go, or roots of other plants. You do not need to separate fry unless you want to make 100% sure that every-single one lives. Typically guppies have more than one fry (4 -> 30), so if you see only one, it means that your fish were stressed, the babies died, or your guppies ate the others. Guppy mothers can also hold off on giving birth for months, and continually use the same sperm to produce new broods over an extended period of time (again, multiple months).

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This is normal for guppies; the males will chase the females, the only way to stop this is to remove the male guppy or be sure you have at least 3-4 females for every male guppy in your tank.

To be sure the offspring survives, you need to get a floating hatchery where you can put the female just before she is going to give birth to her babies.

Please take a look at this article about breeding guppies; this article does contain lots of good information.

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    Thank you, that was a helpful link. I figure should do some additional searching but the truth is I am most interested in the function of the male's aggressive behavior. Can the male tell whether he has impregnated a particular female etc.
    – Buck Thorn
    Aug 21 '19 at 16:56
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    no the male does not know what females are pregnant so they will chase all female guppys,they are a little like teenage boys with raging hormones :) Aug 22 '19 at 5:03

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