soft water

Do Bichir prefer soft, medium or hard water with what ideal PH? I was thinking 3-6Kh with a Ph around 7-7.5? But all comments appreciated.

I have a 20 long paludarium that I set up for my Phragmipedium schlimii orchid and am trying to figure out what other plants I can add? Since the orchid is lithophytic and naturally grows next to and in streams and waterfalls, the set up is rocks partially submerged in water. I might add...