Killi Tankmates

Hi TRT and all you freshwater fanatics, I was hoping I could tap into your wisdom. I have a 20 gallon with a golden wonder killifish, a carnivorous kind. He had other tankmates, but he ate them or they deceased (I believe there is some kind of fungus in that tank because everything i put in there dies except the killi, so I plan to tear it down and start fresh). I have searched the world wide web and have had some difficulty finding tankmates for him that will fit in 20 gallon, won't be eaten by him, or that he won't fight to the death. I had a pleco and loaches, but they died due to the bacteria/fungus, so I know that they will work, but is there anything else? I thought it would be cool to do a planted tank with some interesting fish (not necessarily colorful) that the killi would be OK with. I am open to whatever suggestions you guys can give me. Thanks for your time.