Right now, I have to say Convict Cichlid. A couple of mine laid some eggs and they are very protective of them. It's very interesting to watch.

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55g
x9 Black Convict Cichlids (Archocentrus nigrofasciatus)
x1 Tilapia buttikoferi (T. buttikoferi)
x2 Turquoise Jewel Cichlid

20g
x1 Plecostomus (not positive which kind)
Lots of plants