Gosh any Lake Malawi fish would work.
Well any of the 3 groups, maybe not ANY fish from there.
You've prob seen it before but....
My first suggestion is Africans:
Mbuna - Very colorful, lots of varieties, pretty common and at most LFS's, and can usually be packed in the tank. More in the tank (withen reason) = a less agressive tank towards certain fish. Which means fewer deaths. They spread the aggression out. They can get anywhere from 3"-6" each, fairly large. But with a good many a tank full of these is quite a display. My Mbuna are always swimming, showing colors, dominance stances....very cool tank to watch.
Peacocks - Males are very colorful and "regal". They like to show. Females tend to be less colorful, mostly silver (darkish grey) with hints of color, still to be seen in the tank nonetheless. These usually reach 5-6", females slightly smaller than males. Lots of color varities out there.
Haps - Like peacocks they are pretty bright/shiney
Females being a bit more toned down than the males. These too grow pretty large, some up to 12"....which wouldn't be good for your tank. There are a few however that make for a stunning display. Haps are becoming a favorite of mine. You might look into Placidochromis electra (Deep Water Hap), Protomelas sp. "Steveni Taiwan" (Taiwanee Reef), and Protomelas taeniolatus (Red Empress).