3X1X1.5 is this in meters? keeping a pair of green terror should be fine alone. one thing to always keep in mind when keeping pairs in their own tank, have a divider ready especially when the male becomes ready to breed when the female is not. If the measurements you gave are in meters, the tank is sufficient size for them. If you plan to keep several breeding pairs in that tank, you will have a ton of problems with aggression and most likely will have several dead fishes once frys appears. Also, the least dominant pair will not breed and will most likely be picked on or killed by the other pairs. Good luck in finding your fish!