Any site advising that Congo Tetras need hard water is obviously completely useless. Don't even go there.
Which brings me to a suggestion...bookmark sites that are reliable and use these only. Most of us here use Seriously Fish, which frankly is a s good as it can get for a comprehensive data base aimed at the hobbyist. Other equally valuable sites are for specific fish, like CorydorasWorld, Planet Catfish, Loaches Online. One thing I noticed many years ago is that reliable sites tend to basically agree on data. That is because the data is based on science, not the whims of some half-wit on YouTube.
To the water for Congo Tetras, Phenacogrammus interruptus. First--the African rift lakes are unique. They are freshwater, but this is the highest GH water that the fish species aquarists keep in aquaria are found. There are a couple of exceptions, no matter. I'm being general to save time and words. The cichlidae fish species in the three rift lakes are unique among the tropical species in aquaria. The cichlid fauna of the African rift lakes is a prime example of rapid evolution into morphologically diverse yet closely-related species, and forms an important source of data in the study of evolutionary speciation (Kornfeld & Smith, 2000). This speciation has occurred in the short (geologically-speaking) period of some 15,000 years, with variant forms in each of the lakes. They are the only freshwater species that are fine with salt (as in aquarium or marine salt used for brackish and marine fish). Not a lot, but there is a tad of sodium chloride in the lakes. This is not the case with any other freshwater habitat (again, someone may know of an exception or two, never mind).
The rest of tropical Africa is much like the tropical regions in South America and SE Asia. The water is very soft, and usually somewhat acidic. Congo Tetras come from such water. This species inhabits calm waters of streams and rivers in the Congo [Zaire] River basin. Water parameters are soft to medium hard (4-18 dGH), acidic to very slightly basic (pH 6 to 7.5 but best below 7), temperature 23-27C/73-80F. They need a 4-foot tank, and a group of 10-12 minimum. They love to do what I term relay races, two males side by side from end to end.
