04-30-2009, 11:09 AM
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All of the commonly-kept tetras are characins that in nature are found in groups called shoals. Other characins that are not "tetras" like pencilfish and hatchetfish do the same. In an aquarium they fare best when kept in groups because it is their natural behaviour to be with their own kind--they feel more secure and therefore are less stressed and therefore healthier and less prone to disease. Some do not actually swim around in shoals, others do so more often. Rummynose tetras are in my view one of the best for shoaling; if given enough room (they are active swimmers) they will swim together in a group almost constantly. Cardinal tetras also shoal frequently, as do several of the Hyphessobrycon species like H. bentosi, H. bentosi rosaceus, H. megalopteris (Black Phantom, formerly Megalamphodus megalopteris), etc. All of them will interact with their own species, whether in "play" ("Sparring" is a term I've seen used to describe their mock battles) or obviously when spawning. I've no personal experience with black skirts, but from what you say they may be one of the species that just likes to be in a group with each going more his or her own way.
Be careful of serpae tetras. They can be nippy with other fish. Kept in a larger group this tendancy sometimes wanes a bit, but if you don't want to take a chance having them become mini-terrors in your tank (behaviour like this often causes stress for other fish, and that's when problems like disease appear), there are several other similar-looking tetras that will provide the colour and activity without the risk. The afore-mentioned H. bentosi and H. bentosi rosaceus are prime contenders for reddish fish that are very active but extremely friendly to all in the tank. I have a shoal of the latter in my 90g, and they are constantly displaying to each other and spawning regularly. I also have 98 black phantoms with them, and the two species interact frequently. And of course the male phantoms are forever displaying and challenging each other with outspread fins and almost pure black colouration, quite a site. In a 125g tank you would be entertained with a very beautiful site by a shoal of 15+ H. bentosi rosaceus or similar; years ago I had them in my 5-foot 115g.