I have had three of these fish for about 18 months. I had four until about 2 months ago, when my dominant male went on a killing spree while I was away for the weekend. Since then I am constantly having to remove the other two fish to let them heal from attacks. Up until two months ago, when he killed the first fish, I was very pleased with the aggression levels in my tank... but now he is hyper-aggressive. Things have calmed down in the tank now, but I'm sure it won't last. I'm at my wits end what to do with these fish - the male is very beautiful and I don't want to get rid of him, but...? I have the cobalts in a 310 L tank with rusties, saulosis and yellows, and I'm considering adding some slightly more aggressive fish to balance out the blue. But definitely a great fish if you don't end up with aggression problems.
I have two Blue Cobalts that do well in my 135 L tank with a few other cichlids. As with other people, I've noticed that I've been able to subdue their aggression by feeding them more often (3x per day) and I've actually overstocked the tank as well. I know there are some readers who would gasp at this, but my Blue Cobalts seem to be more tame now that there is a community type setting. They are both a brilliant blue/purple, and I keep my tank set at 24°C, which seems to do the trick. They are very hardy fish to keep, seem to be good for beginners if you can find one that is not too aggressive.
These pages have enough comments to give the reader a basic idea on the topic. Further comments are still very welcome (through the site's contact form) as long as they provide new and/or advanced information not yet discussed in the existing ones.