To further expand on what Star rider is saying, With two fish your bacteria colony will grow to the largest size possible before bacteria begin starving. When you add a couple more, then the bacteria in place is capable of consuming more ammonia than they previously had. additionally the colony size will double within a day or two. and continue multiplying exponentially until it runs out of food. at which time you can add a couple more fish and so on. It's all covered somewhere in the cycling article, but I do realise that is a very large document, and many folks don't have or take the time to read it all.
Dave
Dave