Angelfish and other loners

Most fish have a tremendous amount of 'cousins'. The Neon tetra comes
from a huge family of tetras - danios, mbuna, corys, rainbowfish all have
huge 'families'. Plecos are so diversified that new discoveries are all
simply numbered (ie:L121, L122 etc). Even fish which we might think are
unique, such as the Siamese fighting fish come from a very large 'family'
(Genus Betta). The same for Guppies, Firemouths, Geos and many others we
might think unique. At the other extreme, for whatever evolutionary
reason, certain fish are practically 'alone' in this world (thinking
along the lines of shape, behaviour and niche).

For these loners, was the process of evolution so harsh that every other
variant was exterminated, or perhaps their environments were so
inhospitable to having more than one of something? If their survival was
so precarious, it gives me the feeling that it's only by the slimmest of
chances that we have them here today, and that many unique fishes simply
did not survive long enough to have been seen by modern man. Or perhaps
these unique fishes were so successful that they simply mastered the
niche they found and prevented any competition through diversification.

My vote for the top three unique fish would start with the Angelfish,
which incidentally is classified into more than one species, but among
the experts, this topic is hotly debated and rarely agreed upon. At a
glance, it started with Pterophyllum scalare & P.altum with discussions
about P.eimekei - but now it's P.scalare, P.altum, P.leopoldi and
P.dumerilli.

Experts aside (and I'm sidetracking), these evolutionary variants are
fairly 'recent', and if experts have trouble distinguishing between them,
then perhaps we shouldn't worry about it ;~). In practical terms, 99.99%
of the Angelfish sold today are P.scalare in color/fin morphs (marble,
black, golden, veiltail etc) and the remaining .01% are wild caught
P.altums (silver wild coloration/marking)..imo.

My vote #2 goes to the Pompadourfish (what?). Now more commonly called
the Discus ;~). Here again, perhaps the experts are splitting hairs, and
there are two species Symphysodon discus and S.aequifaciatus and various
sub-species (willischwartzi, axelrodi, haraldi etc), but essentially,
besides slight variations in color patterns and ray counts, it's the same
fish, and there are many color morphs.

Vote #3 is not so obvious. perhaps the Elephant-nose fish or Mono
(Monodactylus argenteus with one cousin, the Sebae). Hatchetfish are
quite unique (Silver or Marble species). Anyone have any other
suggestions?
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