Let's see if some simple imagery works. Look at these two fish.
That pic is in my 6ft tank. Which is 180g. And you say those two fish would fit, and be able to move around adequately, in a tank 8 times smaller than that?
Common sense? Cuckoo! Which part is incomprehensable? I have two parrot fish. I see them, feed them, watch them every day. I see how much they move, how much space they defend, how much space they need. And there is no way - NO WAY - they would fit in a 20g tank and be happy, and move about freely.
Done with this. Maybe for once someone else with a brain could step in and help me out here!