I searched the species database for cichlids that could live in a tank 20g or smaller. The list below is what I've discovered but I wanted to be sure I was correct:
African Butterfly Cichlid
Agassiz's Dwarf Cichlid
Banded Dwarf Cichlid
Blockhead Cichlid
Brichard's Slender Cichlid
Brichardi Cichlid
Cockatoo Dwarf Cichlid
Dickfeld's Julie
Dogtooth Cichlid
Egyptian Mouthbrooder
Fairy Cichlid
Flag Cichlid
Golden Dwarf Cichlid
Keyhole Cichlid
Kribensis
Lemon Cichlid
Macmaster's Dwarf Cichlid
Masked Julie
Nichols' Mouthbrooder
Ocellated Lamprologus
Panda Dwarf Cichlid
Plain Goby Cichlid
Ram Cichlid
Red Zebra Mbuna
Redbreast Acara
Roloff's Kribensis
Slender Mbuna
Steindachner's Dwarf Cichlid
Temporalis Cichlid
Three-Striped Dwarf Cichlid
Two-Banded Cichlid
What do you think?
African Butterfly Cichlid
Agassiz's Dwarf Cichlid
Banded Dwarf Cichlid
Blockhead Cichlid
Brichard's Slender Cichlid
Brichardi Cichlid
Cockatoo Dwarf Cichlid
Dickfeld's Julie
Dogtooth Cichlid
Egyptian Mouthbrooder
Fairy Cichlid
Flag Cichlid
Golden Dwarf Cichlid
Keyhole Cichlid
Kribensis
Lemon Cichlid
Macmaster's Dwarf Cichlid
Masked Julie
Nichols' Mouthbrooder
Ocellated Lamprologus
Panda Dwarf Cichlid
Plain Goby Cichlid
Ram Cichlid
Red Zebra Mbuna
Redbreast Acara
Roloff's Kribensis
Slender Mbuna
Steindachner's Dwarf Cichlid
Temporalis Cichlid
Three-Striped Dwarf Cichlid
Two-Banded Cichlid
What do you think?