Koi definitely are the nicest from above, but Koi eat plants.
Cardboard I have no idea about the compatbility of bichirs with plants but definitely the larger cichlids that are not angels or discus, and most likely the ray would also damage plants.
My thought would be something like gold barbs. They may be small, but they have the same benefit of being colorful from above like koi, but are not going to destroy plants.