12-08-2010, 07:22 PM
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"Schooling" is something freshwater fish technically don't do, comparable to marine fish. "Shoaling" is perhaps a better description, though for most of us the two words probably mean much the same. A shoaling fish--which by the way angels absolutely are, as are discus--means the fish needs a group of its own species to be free of additional stress. The group obviously brings safety, at least to the fish's mind, but many species also establish an interaction within the group that can vary from species to species in its components. Angels set up a pecking order, and having at least 5 of them will ensure no individual is likely to be picked on to the extent of health problems and death.
To the smaller shoaling fish; few will swim together as a group (one difference from marine schooling fish). But there are a few that are fairly consistent at swimming together. Rasbora almost always remain in a swimming group. Some of the danio and barbs do from time to time but no where as consistently as rasbora. Among the characins, the best is probably the Rummynose, all three species. Cardinal tetra also do, but less often. In their native habitat, cardinals have been observed remaining in groups of 5-6 closely within larger groups of hundreds if the water is vegetated; in more open water they remain in much larger groups of hundreds. In the aquarium my experience has been that they remain fairly close, though their swimming is significantly less active than say rummys so the latter are more "obvious" when swimming. The false or green neon, Paracheirodon simulans, also is fairly good at remaining in a group. And many of the pencilfish do, and certainly the hatchetfish. Other tetra species vary their grouping, though they tend to remain fairly close, within "eye contact." Not all of these wil last with angels though.
As for companions to angels, cardinals are a risk. 1077 correctly noted that sometimes this works and sometimes not. It is certain that the risk is there. At 6 inches body length, angels are expert predators, and the "torpedo" shaped tetra often fall prey. Tetra that are more disk-shaped fare much better, and some of the best companions are the species in the Rosy Tetra clade of Hyphessobrycon or the similarly-shaped Hemigrammus species. Several of these are included in our profiles, and it mentions their suitability to angel habitats. Rasbora, if not introduced small to grown angels, might work. Cetainly corys are fine, indeed most substrate fish will be left alone, except during spawning.