I'm going to have to disagree with everyone here. You should be giving your other fish suitable homes before even thinking about adding more fish. At the moment, you have 3 red eyed tetras, 3 x-ray tetras, and 1 cory in your ten gallon which are all schooling/shoaling fish, and you have none of them in schools/shoals. You should get 3 more red eyed tetras, 3 more x-ray tetras, and 5 more cories for now, and play around with the stocking in your tanks. You could put 6 red eyed tetras, 6 x-ray tetras, and 6 cories in your 20 gallon, that would be a busy tank. Or you could keep one of the schools in your 10 gallon and have the rest in your 20 gallon, you have many options, I just think you should be taking care of the fish you already have before buying new ones.