I have a 400 L tank with a community of malawi's in it, including 5 Nimbochromis (1 male and 4 females). My male venustus is about 10 cm long and has a vicious temperament. These are beautiful fish, aggressive, colourful and interesting, but like most cichlids all are different and its more a case of try them and see. I had 4 moorii and though moorii are supposed to be peaceful, they were aggressive and bullied my Nimbochromis almost to death.
I work in a pet store and we have a display tank with African Cichlids. A couple of the bigger fish right now in the tank are the venustus and they seem to be the more aggressive fish in the tanks. But truly a very beautiful fish when seen in a huge display tank of African. I advise someone that when buying this fish he should be observed and consider carefully when added to an already full tank of fish. Keep in consideration that it may be aggressive towards new additions to the tank afterwards, if it happens to be the first one added. When I first started my home tank, over 2 years ago, a venustus was the 3rd fish to go into my tank. I had him for nearly a year and he outlived many other different fish while I basically experimented and learned how to keep a tank, he was hearty and healthy and happy...but extremely aggressive to any new tank mates, and eventually was the reason of termination to my last Oscar unfortunately.
I have to disagree with all the comments regarding the peaceful nature of this fish. I have a 290 L tank which my one venustus controls entirely. I have numerous aggressive fish that are constantly bullied by the venustus. A very lovely fish to watch, I would however not recommend one unless you have a tank which is considerately larger. I will agree that the aggression increases incredibly with growth and maturity.
I have a 680 liter tank with only one mbuna and the rest Haps. I have 7 Venustus (two male and the rest female). They breed like crazy and are not to aggressive, unless they are challenged. The only thing to be concerned with is that they like to play dead and when a little curious baby cichlid comes along, boom. It's over. I have seen that so many times and ususally it's their own fry that they're eating, since they breed so much in the tank.
I used to keep one before I discovered how aggressive they can become after the male gets his breeding colours. I started off with 1 venustus and I thought he was a female because of his female colouring. One day though, I woke to find a half coloured male instead of the female that was there the night before. This comes to show how they sometimes get their colours very suddenly. That's when he killed my 4 frontosas. If you're gonna keep them, keep them in a species tank that has plenty of room to swim in and places for other fish to hide in.
I have three Venustus and they seem to do fine amongst some of my larger Polleni, Frontosa, adult Peacocks. They just swim around the tank hiding and exploring, but I will say this, they eat everything that touches the water. I have a Synodontis petricola catfish and I'm unsure if he's getting enough food, because they eat the sinking pellets before they hit the floor!