20gallons is very small i have 2x 20 gallons and ones a guppy planted disply tank and the other is a grow out tank for my little monster fish which then go into my 180gallon
but been all honest with you you can't really get anythin which is all that agressive for a 20gal mate IMO and i have been in the hobby for 6 to 7 years now.
shelly cichlids. I believe neolamprologus ocellatus is a shell dweller there are several varieties. Some are apparently small enough for a trio to be kept in a ten gallon.
I had a red devil in a 29 gallon when I had no clue what I was doing. He lived for 4 years, which is weird because now I see the water quality and filtration and food and almost everything was poor. Ugh, I'm disgusted in myself. Now he's in a better home.(I hope)