If the population of a species in question was found to live naturally in
relatively small pools, swamps or ponds then the "breeder" would create an
environment in a similar climate and transfer a number of fish, from
various parts of the same pool, swamp, or pond to his newly created pond.
Ponds should be organised in a number of places. After a few generations
someone could net randomly a number of fish to transport to the other
locations, insuring refreshed genitical makeup, while not isolating the
defining characteristics of the (sub-)species. (doubt it'll work in
practice)
But the underlying question is: What aquarist would want to maintain a
population without the benefits of studying or changing the fish to become
"aquarium fish" and not "wild fish". ie, change the appearance and
behaviour.
I believe that few aquarium-breeding wild fish speices can successfully
sustain their original genetics. Not only because of the aquarist, but
also because the aquarium resembles nothing like their natural habitat.
And the worse thing about it is that, government agencies, that could
protect natural habitates and don't, are maybe under the impression that
the animals and plants once inhabiting these environments will be
conserved. But that's bullshit. Once outside the environment in which
they evolved from, they will either change, or perish. It's even happening
without the help of humans, Humans are just speeding it up!