Make sure you get a tank at minimum of 6' long, and a depth of at least 18", however 24" is much more realistic. Also, if you truly plan to breed, I recommend upgrading sooner rather than later and upping your number by 50-100%. Dominant male frontosa typically do not tolerate sub-dominant males. If seen a mpibwe male even beat a female he has bred with to near death. And that was in a 180 and was stocked appropriately. Basically what I'm trying to say is have 9-12 to start with. That way, you can weed out spare males as they develop, and mold the tank to the desired ratio of 1m-5f (less females is ok, but I would not add more than one male).
Also, the peacefulness may change with time. While they are not hyper-aggressive, they're predatory. Go to YouTube and search for videos of fronts eating other fish, I'm sure there are several, as well as forum threads on other threads.
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