And The Winner Is...
by Bob Berdoulay
From Gravel Gossip, Diamond State Aquarium Society, Delaware City
Aquarticles
When playing any game of chance you should be aware of the probability of a certain
outcome. Most of us know that when flipping a two-sided coin our chances of calling it
right are 50:50, 1/2, or 50%. What if we are tossing two coins? What is the probability of
getting two heads? The answer is 25%, since we could have two heads, or two tails, or a
head and a tail, or a tail and a head. Thus we have a 1 in 4 chance of getting it right.
These same rules of chance apply when trying to buy a pair of fish. Except for livebearers
and dimorphic fish, it is often difficult to tell males from females, especially in the
young. Most store employees can’t help in sexing fish. I have been dealing with
angelfish for a good number of years and I’ll be d---ed if I can tell the sexes apart
prior to them pairing off as adults. Most tetras, most danios, etc., are difficult to sex
out. So buying a “pair” of fish is somewhat of a game of chance.
Most hobbyists will tell you to buy a batch of young ones and wait for them to mature and
pair up on their own. That is usually what I do. Well what are the probabilities of
getting a pair when buying more than two fish? The chart below predicts the chances of
obtaining a pair when 2 or more fish are purchased. There is also an assumption that the
tank from which you are drawing the fish contains equal numbers of males and females.
# of fish
Probability of
bought
at least 1 pair
2 50%
3 75%
4 88%
5 94%
6 97%
The formula is: ( 1 - (1/2)n ) x 100 = % (where n is the number of fish)
Of course you can be as lucky as I was in purchasing 8 young Apistos and getting all
males. What are those odds? Around 0.39%.
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