Cichlids. Since I'm confining myself to fishes I have some personal experience with, I can only offer you a disappointingly narrow range of cichlids, South American ones suited to apartment-sized aquaria. To placate you, let me begin with some major Cichlid links, since there are more Cichlid-oriented sites than for any other fish family:
Cichlid links. First of all, the central hub for Web information on cichlids is the Cichlid Research Home Page. This is Ron Coleman's utterly professional introduction to cichlids and the requirements for keeping them in the aquarium, for identifying them and breeding them. There is a wealth of advanced information too on cichlid phylogeny, cichlids in their natural surroundings, and the scientific literature. Note Ron's good advice on being cautious about what you hear on the web. The links from this site will get you everywhere, even to non-cichlid aquarium sites.
At www.cichlid.org the American Cichlid Association publishes the bi-monthly Buntbarsche Bulletin, which is indexed and has a downloadable on-line archive (for members only— could be a sufficient reason to join) .
Of course the combination of mailing-list archives and purpose-written articles at The Krib reflects Erik Olsen's own interest in dwarf cichlids including the "kribensis" itself, now called Pelvicachromis pulcher. Now archived at the Krib: the Apistogramma mailing-list, which ran from Aug 1996 to 2006.
I shouldn't omit a link to The Cichlid Room Companion, maintained by Juan Miguel Artigas Azas. It's an essential central cichlid site and hub of links, with articles by pros like Ron Coleman and Ad Konings.
Eric Gracyalny's pioneering Cichlid HomePage (CHOP), which he maintained from 1994 to 1998, is deep-frozen and archived at web.archive.org. You'll find a searchable database for cichlids, by scientific or by common name, with information you might not find elsewhere, though downloading is glacial.
And there's an outstanding book: George W. Barlow, The Cichlid Fishes: Nature's Grand Experiment in Evolution (Cambridge MA, 2000). This is written for the curious layman rather than for the cichlid hobbyist, who is already well served in specialized Cichlid books from the aquarium point-of-view: see a more extended review.
Though Cichlid fossils peter out before the Cretaceous, leaving researchers with a handful of detached spines and ear stones (otoliths), the basic familiar Cichlid type had become established in the southern-hemisphere supercontinent of Gondwana by about 150 million years ago. How would one know that? Modern distribution of Cichlids provides the clues. First to rift off Africa were India (which has a few Cichlids of the genus Etroplus) and Madagascar (which also has some unique and conservative Cichlids), along with the Mascarene Platform (a slowly sinking continental fragment that carries the Seychelles Islands on it). Then, about 84 million years ago, further rifting tore India and the Seychelles from Madagascar and began their drift north, carrying the Indian Cichlids. PBS' "Nova" did a program on the Seychelles, and a webpage with a relevant article "Seychelles Through Time" has a cool animation of the breakup of Gondwana that would make Old World Cichlid distributions clearer.
Apistogramma and other South American dwarf cichlids.
The Apistogramma Study Group guides captive breeding programs and publishes conservation studies, academic research, breeding information etc. etc in the definitive Apisto newsletter, well-titled The Apisto-gram. The organization is dedicated to advanced hobbyists and ichthyologists worldwide. Its members are your best sources for trading out-of-the-ordinary Apistos you won't find at the LFS. The Study Group reaches beyond Apistos to all dwarf (less than 4 inches) Neotropical and West African cichlids. At their website homepage, click on "Phylogeny" to see Mike Wise's recent revision of his much-reprinted 1990 article on the genetic relatedness of the Apistogramma species-groups.
Sven Kullander's authoritative page on South American cichlids offers the best science from the master, at the Swedish Museum of Natural History.
"The World of Apistogrammas" is Frode Roe's handsome, plentifully illustrated tour, with expanded descriptions of species.
Alf Stalsberg's vivid and authoritative page featurs Lætacara and Æquidens spp.
David Soares is half of the energy behind Soft Water Dwarf Cichlids, featuring Apistogramma and other dwarves.
Apistogramma.com is a membership forum that discusses other cichlids as well as Apistogrammas.
Further links specific to Apistogramma are found under Apistogramma agassizi.
Other Cichlids. The Cichlid Scene: The big Central American cichlids in unsurpassed condition, kept the way they need to be housed— in mammoth tanks. Good cichlid stories, resources, well-chosen web links, excellent pix.
Vinod (Vinny) Kutty's website is "Mostly Cichlids," especially the pike cichlids, Crenicichla. He collects them, breeds them and makes outstanding photographs. He also writes about them with wit and style.
The Armkes, in New Braunfels TX, especially import Tanganyika cichlids and other Lake Tanganyika fishes. Here are the shell-dwellers.
Cichlids I long to keep.
Lamprologus meleagris. A shell-dwelling dwarf from Lake Tanganyika, unusually small and unusually colorfully patterned, at least by Lamprologus standards. They are a bit bolder, I understand, than the shy N. brevis. The graveyards of Neuthauma snails form home turf for communities of shell-dwellers, but mine are going to be living in the shells I can get from a gourmet deli, meant for serving escargots. There is an article on these famous shell-dwellers by Peter A. Lewis, "Shell-Dwelling Cichlids of Lake Tanganyika," reprinted from the Buntbarsche Bulletin archived at the Cichlid Room Companion. More Lamprologine shell-dweller postings are at TheKrib. And Matt Pederson of The Cichlid Factory, Chicago, has a dependable and well-written introduction "Desktop Cichlids: the Shell-Dweller recipe."
Pterophyllum altum. The difficult and beautiful "Altum" Angelfish.