Lemme try my hand at this.
Every fish, regardless of species, size, or geographical location, releases hormones. These hormones are part of a system to aid natural selection: they stunt the growth of growing fish nearby, thus reducing the amount of competition for the larger fish (bigger fish=more hormones). This, obviously, plays a very little role in nature because of the sheer volume of water present. But in a tank, especially a tank that is too small for the fish you have, the fish's hormones begin to work quite effectively. The major problem with this is that the hormones affect every fish in the tank, stunting growth and shortening lifespan. Although your fish looks healthy, its not, and it will ultimately die a premature death (thus wasting your money). In short, if you want more fish: get a bigger tank, or pick a better species of fish
heck, ill break it down even further for you:
porcupine puffer + 55gal tank + fish of any other type = you - your money + dead fish