I would agree with either an african tank or planted dwarf cichlid tank, but since you claim to be a minimalist by nature you probably only really want one fish. Since you have dark gravel, my thoughts on a cichlid would be Herichthys carpintis(Pearlscale Cichlid, Green Texas Cichlid, Blue Texas Cichlid) . Only grows up to 20 cm, which is smaller than the 30cm Herichthys cyanoguttatus (Texas Cichlid, Rio Grande Cichlid). At 20cm it shouldn't effect water quality.
Do clients ever visit?
Do clients ever visit?