Mixing Mouthbrooders from the African Lakes. 

Keeping Malawi and Tanganyikan cichlids together works fine for juveniles, but is generally disastrous with adults. Malawi's are hyperactive, less shy and more territorial and continiously aggressive, while tangs. are quieter, less competitive and aggression is directed towards conspecifics. [own species] Diet is also a problem.
An example is keeping Frontosa with Aulonacara, both will eat the same food but while the fronts hide from you the aulonacaras get all the grub, so you add more which goes the same way. This is doubly bad as the fronts stay hungry and the rest overeat. Result skinny frontosas and fat aulonacaras leading to unhealthy fish and absolutely no breeding. If you must keep these species together, feed the aulonacara by day and the fronts when the light goes down. Frontosa are active feeders one hour earlier and one hour later than most diurnal species.
Water chemistry is different, Malawi is calcium rich while Tanganyika is magnesium and Potassium rich. Malawis are generally more tolerant of poor water quality.

Other observations.

Most boring fish:  Adult Frontosa and Discus, <i>[young are great]</i>
Most difficult mouthbrooders to strip young from: Ahli Leptosoma Frontosa and Careuleus, will almost die rather than be stripped, whereas Moori will  spit in a crowded tank if you as much as look crooked at her, then the young dive to the gravel making it impossible to rescue them, all are soon eaten by tankmates.
Most underrated fish: the beautiful tanganyikan featherfins and sardine cichlids which are always displayed badly in shops.
More observations to follow.
Derek doyle