If we are talking about the Gold Saum green terror, IMO 75 gallon at the smallest. At the least you want a tank at least 4ft (48 inches) long and at list 1 ft (12 inches) wide. This will give them swimming room. a 40 gallon breeder (36 inches long) way to small to give even a female green terror room to swim about. Think about if a fish is 8-10 inches that only give the fish 2 feet to swim

, Adult males would be cramped in a 55 but its possible. i have seen males 11-12 inches long from head to tail and females just as big. so a 75+ will give them more room, which keeps them happy , wich means less stress for them.