On the speciation side of this question... Speciation typically won't just happen in the short term solely because a population is separated from its ancestors. If there are no environmental pressures for selection to work on then any evolutionary mechanisms (mutation, sexual reproduction, ...) are most likely going to keep selecting for the same features. Given enough time there would likely be some divergence but without pressures to evolve it would be a very slow process, and as others have noted the timeframes for most examples of isolated inland fish we see are pretty small.