General Aquaria Discussion • Odds of Getting One Male Fish

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Odds of Getting One Male Fish

Postby Benaiah » Fri Apr 23, 2010 9:01 am

I'm setting up an all-male tank. As a result, when I buy juveniles, I need to buy enough to ensure good odds of getting at least one male. I saw the article on the odds of a male/female mix, but have not found anything that gives me the odds of getting just a single male. Perhaps it's just a simple statistics question...what are the odds of purchasing at least one male from a group of 2, 3, 4, 5, or 6 unsexed juveniles?
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Postby cjacob316 » Fri Apr 23, 2010 9:29 am

50%

no matter how you split it, each fish only has a 50% chance of being a male, buying 6 fish does increase you chances of having a male, but it also increases your chances of having a female, so it evens out to about 50% chance of one sex or the other. increasing numbers doesn't increase odds of a certain result, it increases the number of chances you have to get favorable odds, it's like the lotto, one ticket or 20 tickets it's still the same odds, just 1 opportunity vs 20 opportunities to have them in your favor

at some point it becomes cheaper to buy a 3-4 inch colored male
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Postby Fogelhund » Fri Apr 23, 2010 10:15 am

cjacob316 wrote:50%

no matter how you split it, each fish only has a 50% chance of being a male, buying 6 fish does increase you chances of having a male, but it also increases your chances of having a female, so it evens out to about 50% chance of one sex or the other. increasing numbers doesn't increase odds of a certain result, it increases the number of chances you have to get favorable odds, it's like the lotto, one ticket or 20 tickets it's still the same odds, just 1 opportunity vs 20 opportunities to have them in your favor

at some point it becomes cheaper to buy a 3-4 inch colored male


No, it isn't like the lotto at all. Statistics doesn't quite work that way.

With one fish, the odds are 50%

With two fish, the odds are 75% you will end up with a male.

The probability that you will end up with a male, out of six fish, is 63/64 or about 98.4375% of the time.
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Postby Benaiah » Fri Apr 23, 2010 11:55 am

Thanks -this is why I didn't do so well in college statistics. Digging around, I found that this was a "binomial distribution" problem...and then found a calculator to do the math. Here's what it looks like (I think).

Probability of one male fish among

1 fish- 50%
2 fish- 75%
3 fish- 87.5%
4 fish- 93.75%
5 fish- 96.875%
6 fish- 98.4375%

Based on this I'm comfortable buying 4 juveniles - that at least has 9/10 chance of getting a male. Of course, the actual population may be different and lead to different results, but at least it's a theoretical starting point. :o
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Postby Pagan » Fri Apr 23, 2010 1:22 pm

Good to know. Thanks for asking this question. :wink:
Pagan :)
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Postby oldcatfish » Sat Apr 24, 2010 9:12 am

Actually, it's even more complicated than that.

With some species (such as S. Fyeri--the electric blue Hap from Lake Malawi), the spawns have a higher male to female ratio. Adding to the "problem," the fry tend to grow unevenly, and they can be cannibalistic. This is just one example...with other species, the water conditions of the spawn affect the ratio of male/female. So it may not even start at a 50/50 anyway.

In most cases though, if you by six---you'll get at least one of each sex.
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Re: Odds of Getting One Male Fish

Postby DaveGould » Wed Apr 18, 2018 3:57 am

Can we make this a real world scenario? What is better?

3 unsexed juveniles for $25
1 guaranteed male for $18, in the chance that you receive female ( fish doesn’t show after time), breeder will replace. He pays cost.

More hassle than $7? Is worth saving?

Curious others thoughts and experiences
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Re: Odds of Getting One Male Fish

Postby Cyphro » Wed Apr 18, 2018 4:04 am

Depends on what you want. If all you really want is one male, get the one male. If you want a breeding group then get some unsexed ones. I would never get 3 though, that is a wacky number regardless of your purpose. I would get minimum of 4 and probably get six if I want them to breed. Having a mediocre or bad male is also not worth much for that purpose.

Of course unless you are sure they look great then it is probably not a great idea to plan to have them breed. Otherwise you have even more meh fish spawned. Breeding works best if you can find one really good male and build on that, and that is all about luck and effort.
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Re: Odds of Getting One Male Fish

Postby lambretta » Sun Nov 17, 2019 6:08 am

wouldnt those odds would also apply to females so you could reverse it and say that there is a 98% chance that there will be a single female in the 6 fish
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Re: Odds of Getting One Male Fish

Postby Dooner » Tue Nov 19, 2019 6:43 pm

Just curious, what will you do with the females you end up with and what will you do with multiple males as that may present aggression issues? Those are issues I am facing at this time.
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Re: Odds of Getting One Male Fish

Postby DJRansome » Tue Nov 19, 2019 6:48 pm

You rehome females if you are going all male, and males if you have extra.

Do not rehome females if you cannot ID the separate species after mixing look-alike females, like peacocks. In that case, you would not mix them in the first place in the same tank.
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