Shell dwellers are indeed a great option! Given the dimensions of the tank I think that might be the best way to go. That said, if it were me, I'd probably look into also adding some fish that will occupy more of the top of the water column. Maybe some cyprichromis?
You might also consider calvus and compressiceps from Lake Tanganyika. They are some of my all-time favorite African cichlids. In my experience, they aren't crazy aggressive (at least by African cichlid standards!), but they are SUPER slow growing.
I'd also consider some of the dwarf Mauna species. The length of the aquarium isn't ideal, but you might be able to get away with it with enough rock-work. Pseudotropheus demasoni are some of my all-time favorites, but they are incredibly pugnacious.
Just some thoughts!
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