Yep, mbuna as brandon said are a species of highly adapted cichlids native to Lake Malawi and the name literally translates as rock dweller. They have highly flexible spines and often specially adapted mouths for grazing algai/aufwuchs off rocks.
In the wild, the lil buggers claim a good old territory (the males) and try and attract females to breed with - they are highly, highly aggressive to conspecifics/other fish generally but to the former more so. In the aquarium you manage this by careful selection of species for compatability (basically nothing too gentle - relatively - with the more aggresive species as they stressed badly and outcompeted for food) and by seriously overstocking so that no one fish actually claims a territory - that is what gives this lovely bright display of many fish so typical of an african tank.
Four foot tank is considered minimum appropriate size for a manageable stocking.
Also, you stock 1 male with say 3 females of each species you keep. A lone female will get chased and harassed potentially to death. They breed very readily, and hybridise happily with other species.
For some general reading around stocking plans
www.cichlidforum.com has some nice ideas (I wouldn't agree with them all but they are a handy starting point).
In your tank - 72 Galls - you could have a marvellous display of mbuna...say 35 fish or so at least (with the right filtration - you need at least a couple of great cannister filters on there because of the overstocking - a little more).