Some folks advise to overstock Mbuna to disperse aggression, but this is really the wrong way to handle aggression. Even with sufficient filtration, Nitratras are at the end of the Nitrogen cycle, so more fish=more Nitrates, and the only way to reduce Nitrates effectively is by doing PWCs. Aggression with Mbuna can be controlled by selelcting the proper species to coexist and have a sufficient sized tank with hiding places breaks in line of sight.
Peacocks, however, open-water swimmers. Overstocking a Peacock tank is particularly not a good idea and hiding places may not necessarily help aggressionin all cases.
Check out http://aqadvisor.com for a GENERAL guideline on how many fish you can support in a given tank/equipment. It's not fool-proof, but it's a decent starting point.
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They also overstock because it works. Some fish as individuals are hyper aggressive. I had a compressiceps that literally removed 2 pelvic fins fins completely from one of my fish. If you don't care about a little work water changes are no big deal, at least to me.
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