20 gal long? Not 20 liter.. Do a school of something.. For instance with my apisto's in my 29 I have 8 rummynose tetras and 6 hexazona barbs... The rummynose are all over the tank while the barbs stay lower.. Makes for a nice scene with everyone one their own zones.. You could prob do a school of 8-10 smaller (1-1.5") fish...