The term "breeder tank" is a misnomer when taking about any, but the smallest cichlids, and was probably coined when most people simply bred small live bearers, zebra danios, or other community fish. In nature most cichlids defend a 4ft square area (240 gals), and like Ryan said, a 150 is minimal if you want to keep a pair and breed them naturally as a pair, to term.
Just about any cichlid will kill a mate in a 40 breeder, and the remaining parent will proceed to eat eggs or fry at the slightest twitch or shadow.
If any of my cichlids decide to spawn in something small, such as a 55 or 75 gal, as a rule, I leave only one parent with the eggs and fry (usually the female).

Just about any cichlid will kill a mate in a 40 breeder, and the remaining parent will proceed to eat eggs or fry at the slightest twitch or shadow.
If any of my cichlids decide to spawn in something small, such as a 55 or 75 gal, as a rule, I leave only one parent with the eggs and fry (usually the female).

