120 Gallons is a LOT of area for these guys. You should have no problem keeping a fair sized group in such a set up. Especially if you aquascape upward as well as across, ( meaning providing a good base of rockwork for territories and hiding spaces. ) As a rule Cichlids are all greedy to some extent. Whether it's territorial, breeding, or food greed the trait is there. Will convicts set up huge territories? If given a chance yes. What cichlid would not. But the spacing is determined not only by size and dominance of the breeding pair but also by the available space, the visual breaks in the habitat ( Rockwork that hides fish from easy view of eachother ) and of course the competition for space. If you take twelve pairs of convicts that are all the same size and place them in a 120 gallon with a good layout, you will have 24 breeding fish who will squabble but will accept that they MUST stay in their territory to avoid take-overs by neighboring pairs. Another technique popular with some European hobbists when trying to breed cichlids who pairbond and raise the fry is to have an odd fish out that is a different species. This fish is called an "agitator".
The premise of the "agitator" method is that by keeping ( for example ) three pairs of large aggressive breeders with a larger tough fish of a different species, it focus's the defenders on keeping the agitator away from the spawn and difuses energy spent on other pairs or the potential of consuming their own fry out of bordom, nervousness, or whatever other causes. The agitator needs to be a bigger and fairly tough species to survive the abuse of cruising over the territories of protective parents. It may eat the odd fry now and then, but generally agitators are good at keeping cichlids focused on parenting and not on territorial disputes or other pairs of their own species.
Best of luck to you. Oh and BTW Convicts will breed like mad! They are good parents and spawns grow in number as the parents get larger and more mature. Something to bear in mind when trying to decide how many to place in your tank. It's not a matter of IF, they will breed, only WHEN! The more you keep the greater a baby factory you will have on your hand. Take care.
- Erik -