07-22-2010, 08:06 PM
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Aside from previous points, both danios and tetras are shoaling fish that should only be kept in groups of 6 or more. A 10g is not large enough for all that. And not for danios anyway because they are active swimmers that need (deserve) space to swim, at least a 24-inch tank. Cardinals are not active and like quiet plant-thick tanks, but six of them would fill your 10g and with your water parameters I would not suggest cardinals as they are sensitive and require soft (very soft) acidic water to be healthy, and I would recommend a larger tank of 24 inches.
A male betta deserves its own space, with perhaps some bottom fish, so that's one option. If you leave out the betta(s), there are some very interesting fish for a 10g. Given your basic water, the Celestial Pearl Danio (sometimes seen as Galaxy Rasbora) or Endlers livebearers come to mind. With softer water the dwarf rasbora fishes (Boraras brigittae and similar species), Ember Tetra, pencilfish like Nannostomus eques or Nannostomus marginatus.
The shaded fish names will take you to the profile for that species if you click on it.