Back when I had my 46 bow set up, I had an Angelfish. I purchased him when he was very young around when I actually first joined the forum here.....and two years later he was MASSIVE. He was a really fun and interesting fish, very attentive and curious and smart....I had to train him not to bite me when I did water changes though. xD
He was a decently active fish though, quite literally RULED my entire tank. Personally, I don't think a 20 tall is large enough....something like a 29-40+ gallon I think would be much more ideal long-term.
Also, though I'm not sure of the exact hardness level, when I had my Angel I lived on hard well water with a very high PH. He seemed to do just fine, always bright and colorful, never had any problems.