The dissolved mineral content is generally negligible and the pH can be as low as 3.0 or 4.0.
http://www.seriouslyfish.com/species/betta-uberis/
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can survive under such conditions such as Microsorum pteropus, Taxiphyllum barbieri or perhaps some potted Cryptocoryne spp., and a few patches of floating vegetation would be useful to diffuse the light entering the tank.