Probability of a Pair

Some fish like livebearers can normally be sexed quite easily but some species like angelfish have no external characteristics that allow them to be sexed. Juvenille fish are also normally hard/impossible to sex. Obtaining a pair is often preferred by most aquarists, so how do you improve your chances of getting a pair? Well it's all probability, more fish = more likely to get a pair. For those who are interested the probability can be calculated using the formula;

100 * ( 1 – 0.5^(N-1)) = % chance of a pair
Where N = number of fish

Therefore, if you bought 3 fish the probability would be 75%.

3-1=2.
0.5^2 (0.5*0.5) = 0.25.
1-0.25 = 0.75.
0.75*100 = 75%.

Most people recommend purchasing 6 fish to give yourself the best chance of a pair this is because buying 6 fish gives you a 97% chance of a pair.

Hope this helps,
Sam