Planning on buying a 20 gallon tank

Hello everyone on this sub! Since it’s Christmas I was thinking of getting a 20 gallon tank and turning it into a cool aquaponics type system. I want to first start with having the tank it’s self flushed out. I was wondering if I could get some tips on species of fish and plants that would work with the size. I wanna also focus on biodiversity and a fresh water ecosystem. Thank you very much in advance.