The rule I use for for cichlids in my tanks, is 10 gallons of water minimum, per inch of adult fish, due to territoriality and aggression issues. In nature a pair on convicts might normally guard an area of about 200 gallons
So for me, a 60 gallon tank is bare minimum for a single pair where the male gets 6".
The old adage of an inch per gallon, only applies to fish like guppies, danios, or neons and the like, not cichlids.