arowana water hardness

aro prefer soft n a bit acid but got no problem with hard water

South American species will certainly appreciate low hardness, as will most Amazon fish. Based burely on their habitat I'd say the Scleropages Spp. would prefer it on the softer side too, not sure about the African though.

Just watch your pH if you have no hardness, it will fluctuate very easily and if you have wood in your tank it could crash and drop very low. A small amount of crushed shell or coral will help buffer it against extreme pH swings with out pushing it too high.