This post was in response to mine, so I will respond to the questions. Other members have said much the same thing, but I do not want you to think I am ignoring you.
Yes angelfish are cichlids, but they live in groups, not in the hundreds like characins, but they are found together in their habitats. The need for shoaling in this species was studied, and the scientific paper can be accessed free if you want to see the facts on this species:
Angelfish develop an hierarchy among the fish living together in an aquarium. This plays out very differently in an aquarium than it does in the natural habitats. The dominant male in a group will decide who is and is not welcome. In the aquarium, a fish that is not welcome in the shoal will be hounded, usually to death in time. In the habitats, such a fish would simply swim away, and find his own group or whatever. If a pair forms--and angelfish must choose their mate or it will not last long--the tank size will result in the other angelfish being attacked. This does not always mean physical aggression, it can be the chemical signals called pheromones that fish use to communicate. If you have two, three, four or usually even five, this is asking for trouble. A single angelfish can survive, but that is not the same as thrive, because it is being kept contrary to what it expects which is programmed into the species genetics; it may or may not exercise its dominance over the tankmates.
Blue and gold gourami are varieties of the species Trichopodus triochopterus, there are many varieties developed but they are the same species, and unfortunately this is one of the most aggressive of the small and medium-sized gourami. A female one member had killed every other fish in the tank. I stood in front of a tank in a local fish store one day and witnessed two blue gourami (same species) circle a neon tetra and within seconds the tetra was gone. Store tanks are not permanent home tanks where we can research the fish species and avoid trouble.
This tank is probably 12 inches in width, and that is fine for the fish we are discussing. I had 40 wild caught cories in my 40g tank, along with a group of 12 rummy nose, a groups of 7 pencilfish, a whiptail (Rineloricaria parva, the smaller species). This tank ran successfully for several years. It is not a case of numbers verses volume, it is a case of understanding the needs of a species and providing that need.
Columbian Tetra--an excerpt from my online profile of this species (Hyphessobrycon columbanus):
Compatibility/Temperament: Peaceful, though some report it may nip fins. A group of 9+ may lessen this tendency; a shoaling fish, it needs to be in a group. It should not be kept with sedate species such as angels, discus or gourami due to its boisterous activity and aggressive feeding habits [see comments under Diet] but only with similarly non-aggressive characins, rasbora, small barbs, danio, catfish and loaches. A good dither fish for dwarf cichlids.
Diet: In it habitat, feeds on small invertebrates. Accepts prepared foods; frozen and/or live bloodworms, daphnia, artemia. A hearty and greedy feeder, it must not be overfed; it has been observed taking food out of the mouths of other fish.
Diet: In it habitat, feeds on small invertebrates. Accepts prepared foods; frozen and/or live bloodworms, daphnia, artemia. A hearty and greedy feeder, it must not be overfed; it has been observed taking food out of the mouths of other fish.
