This isn't how it works. Yes, fish can and do hybridize in the wild, but this is not the mechanism for new species' development.
Mutations occur naturally within populations. Some are harmful, and those individuals that express that trait will die. Some are beneficial, and those individuals that express that trait are more successful than others, and in turn, reproduce and pass on the desirable trait. Over a great period of time, the successful mutation is passed on and on.
New species can develop when populations are isolated from one another. Environment selects what traits are successful and what traits aren't. When these populations are isolated from one another for a long enough period of time, they become distinct species.