it always strikes me that people resemble the kinds of fish that they own... there's
a previous thread about this; but here's my two cents (please read this as a tongue-in-cheek comparison, not a perfectly accurate analysis

):
Livebearers - Overall very friendly and chatty; enjoy daily interaction and asking for advice (although this advice isn't always taken). Love the bright colors. Can be nippy but overall are quite mellow. A surprisingly large overlap between guppy and betta keepers.
Bettas - More pugnacious than the livebearer forum, but that's mostly due to the individuality and protectiveness of the involved; generally only lashes out in defense of proper betta-keeping. Delighted by strong or unusual colors and finnage. Occasionally confuses the needs of other fish with those of bettas; this seems to occur most often when introduced to fishkeeping through a single betta. Most collect bettas and feel compelled to purchase every betta they see.
Cyprinids, Characins and Atherinids - Typically very harmonious with bursts of activity. A few shark-keepers wander through occasionally with questions about compatibility; these encounters often result in personality clashes or even violent disagreements when the inquirer has insufficient room in his tank. While not as chatty as other forums, there is a strong underlying sense of community based on common interests and understood positions. Occasional panics over potential illnesses, predatory/territorial threats and breeding concerns.
Gouramis and Anabantoids - A calm and quiet group. Interaction seems to mostly occur in discussions regarding territory needs, and inter-species compatibility. Altercations rarely escalate to the levels seen in Bettas or the Cichlid forums, but they do occur. Captive breeding questions are relatively uncommon and expressed interest is mostly theoretical. Typically found with slow-moving and well-planted tanks.
Oddball's Institute - Here be monster-keepers! An eclectic group with a variety of interests that enjoy the new and the different. Often find their interest most strongly piqued by wild-caughts; many are enchanted by long worm-like fish. Open to speculative conversation, but be wary of ill-informed questions however! Some members may bite and a few of them have teeth. Inquiries regarding rays are welcome but often short-lived. Questions regarding shrimp and other crustaceans are generally eaten alive unless removed to the Invertabrates forum.
Brackish - An outgrowth of Oddball's, it contains several overlapping regulars. Not the most widespread of groups, but occupies an important niche. Tends to field regularly the same sorts of questions. Many visit once in a while, but only a few spend a significant amount of time in the forum.
African Cichlids - There's always a moderate level of activity in the Africans forum, a vibrant and often colorful group. While discussion of the Lake cichlids dominate, there is the occasional question about miscellaneous W. African cichlids (usually regarding kribensis breeding). Great pride is displayed in each individual's tank and stocking; advice is only sometimes asked and often unwanted. Loosely moderated, the forum seems mostly self-regulated and overly aggressive comments are generally subdued by one or more of the more dominant members.
New World Cichlids - Readily divided into smaller and larger cichlid keepers. Smaller cichlid keepers are the more uniformly vocal and a few inquiries on the finer aspects of dwarf cichlids can be expected daily. The owners of the larger and more violent cichlids seem to lurk more often than not, responding only when they feel their expertise is called upon. The most colorful language (short of bettas) is found here. Sometimes a newcomer with more savage interests will appear, but they are generally rebuffed when the group grows tired of their aggression. Most members find a feeder-only or people-food diet to be unhealthy.
Catfish - An astounding variety of individuals, ranging from the community-owner with cory cats to the monster-keeper with tiger shovelnoses. Dominated by questions about the specialized bottom-dwellers of the Americas, the Catfish forum is a wealth of information and its list of regulars is continually growing. Most members are quiet sociable and mellow; Catfish could easily be characterized as the friendliest group. However there are some members with big mouths and bad tempers, so as always, tread carefully. Be sure to look at both the avatar and name before identifying members, as these can be quite similar sometimes--leading to confusing misidentifications.
Hybrid Fish - A completely unpredictable and often controversial group. Best undertaken with an open mind and flexible imagination, or simply left to the experts.