Once a pair has formed, the others wont last long. All you can keep is one pair in that 20 long. Breeding cons are one of the most aggressive fish for their size.
agree with most of the others. I don't know who told you to keep them in a 3 to 2 ratio but they were wrong. convicts are pair-forming cichlids. they form a male/female pair and will spawn. they then will fight off anything in the close area and if they don't have enough space, they may even fight with each other. personally I don't think a 20 long is not big enough for a pair of cons for very long and it's definitely not big enough to keep them long term.
cons can reach 4-6" easily so their potential adult size alone makes them too big for a 20 long. they need the extra height that a 29 gallon provides. i know most cichlid people will say that height isn't that important when it comes to cichlids....and to some degree that's true, but there should be a minimum of at least 17-19" in height.
I say this because, when a pair forms, typically the female stays in the nest guarding the eggs and then fry while the male hovers above guarding the general area. if there is no where for the male to hover above then he remains too close to the nest and this makes the female mad and she may try to chase him away.
so all that having been said, I wouldn't try to keep a pair of cons in anything smaller than a 29 long term but even that, imo is too small for a life-long tank for them. I'd look for something in the neighborhood of a 30 (36" x 12" x 17") or bigger as a good, long term home for them.
here it is the truth no matter how you feel this is the truth. i bestow upon you the knowledge i gained owning this breed. for starters you have an extremely small tank for your number of fish.what is going own that your fish is fighting for territory thats rightand it really dont matter cuz in a few months thatcouple you have that has paired off will multiply trust me i too started with several in a 55 gallon the dominant male killed off the other male only to make room for his fry. then once his fry get big enough to eat his new batch he will try to kill as many of them as possible. if you think i am lying then research this . i wish you and yours the best of luck.