Re: Why can't different kind of fishes get along when living in the same tank?

You state that you are observing "a population of one species", but then
ask "why can't different kind of fishes get along when living in the same
tank." It is therefore unclear if you are asking about a population of one
species or multiple species. 

Different species of fish display varying amounts of aggressive,
territorial, schooling and mating behaviors, either within others of their
species or other species.  These behaviors can be exaggerated when fish are
at high densities, e.g. in crowded fishtanks.  Since your question does not
describe the behavior of your fish in any detail, it is difficult for me to
suggest the cause of that behavior.  

Why fish don't appear to "get along" in groups similar to humans?  Until
fish neurobiology catches up with that of primates, only the fish can
answer that question.  However, there are lots of human beings who can't
get along well with other people, so maybe the behavior of your fish isn't
that uncommon.