In general, the fish you are keeping, all get to large for a 20 gallon. The goldfish, whether it be comets or koi, will get to large. In addition, they produce tons of waste. The angels will also get too large. Personally, I would not keep them in anything less than a 55 gallon. However, a 20 gallon is fine if you are seperating them in order to try and breed them.