Good Medium Sized Cichlid Wet Pets

[Hybridfish7]

In my personal opinion, solo male amatitlania sajica, cryptoheros cutteri/spilurus, any of the larger wild convict(/siquia) locales, or any of the cichlasoma species.
However if you don't want cichlids, then miurus puffer, any of the larger carinotetraodon, some of the smaller gymnotids, if you can find one then clouded ghost knife (nebulosus), or just an opaline gourami.

 

[Rass]

I agree with the amatitlania and cryptoheros species as a possibility. Rainbow cichlids are great as well and don't get very big. Herotilapia multispinosa.

 

[Deadeye]

No mbuna should be with others in a 30 gallons. My auratus and red zebra ran a 36 and made life hell for the rest, with the auratus eventually looking the zebra.

Puffers and labyrinth fish come to mind first.
Solo peacock eel with dithers as well.

Edit, killing not looking.