Quite true. Our profiles list the external gender differences if these are known; often it comes down to the females being "rounder" or something.
On the aggression, this depends upon species. But generally speaking--generally--males will be "territorial" to some extent while females are less likely to be, though there are always exceptions. Male territoriality can be quite aggressive as in many cichlids, some gourami, some loaches, some catfish, etc., where physical damage may occur to subordinate males and sometimes females; or it may be as "harmless" as many tetras, rasbora, danios and barbs and consist of periodic pushing and shoving or what may seem like a game of tag, though some species are more physically aggressive than others. This is why shoaling fish should always be in a sufficiently-sized group and in adequate space, so that the dominant male's "aggression" will not be directed to one or two fish but spread around. With most of these shoaling fish, you would hardly notice the "pecking order" unless you are aware of the behavioral aspects of the species and very observant.
With some species such as cichlids and gourami, keeping several females to one male does work to ease the attention paid to any one fish. This occurs in livebearers too, and some characins.
This sort of behaviour occurs in many other animals; even in mammals--wolves for instance live in a pack with an alpha male that is the boss and the other males are subordinate. You see it in insects too. All part of life.
Last edited by Byron; 07-25-2010 at 01:43 AM..