Boy do these fish have attitude! I have had a 200 L mixed cichlid tank for 3 years, and this fish is in total control of the tank. He is not the biggest, but he is the baddest. He will relentlessly chase a female (unknown to me) forever it seems. I've watched him battle one of my plecos and actually suck on him and rip like a dog would. He is feared by all the fish in the tank. I wouldn't dare part with him.
I have had my Cobalt for about 5 years now, he is currently about 15 cm long. He is definately the dominant fish in my tank. His tankmates are a little unusual - a Green Terror, a Jaguar cichlid, a Jack Dempsey, a Pink Convict couple, an Electric Blue Alhi, a Jurupari, a Pike Cichlid, a Rainbow Cichlid, a Flying Fox, a 2 Striated Botia, a spotted Raphael catfish, a Red Emperess, a Yoyo loach, and a freshwater lionfish. Despite this odd mix, there is little or no aggression whatsoever between any of my fish. But they all get out of the Cobalts way when he comes around.
I have two Blue Cobalt Cichlids that helped introduced me to my Aquaholism. They were the first (two) ever fish I bought and survived all the newbie mistakes I’ve made along the way. The two Blues lived through one less than textbook tank cycling in the beginning, my over feeding, under feeding, frequent redecorating, over and under cleaning, and multiple tank moves. One of the pair seemed to be quite upset at my antics and lost nearly all its color, turning almost white for quite some time. It took a good while for the color to return, but that was a few tanks ago. The pair is enjoying the good life finally in a 55g Corner Bow with a few of my “favorite” or prized fish. Both are extremely active explorers. They tunnel under and around a large coral lace rock centerpiece creating their own little caves one rock at a time. Neither seems overly aggressive in a physical sense, never having injured another fish but they do chase a bit on occasion. Both seem to have picked out spots to claim and defend but retaliation is usually nothing more than some “flexing” (which looks like a seizure) and a quick chase. Maybe they’ll catch something sooner or later, but they’ll always get cut some extra slack being my “first” fish.
Cobalts are great cichlids. I have had one for almost seven years in a 220 L tank. It is about 16 cm long. It actually had babies this year for the first time, believe it or not it crossed with a Labeotropheus trewavasae "Red Top"! I didn't think it was possible, but it is a cool looking fish. When the five babies were born, the cobalt got really aggressive and killed 3 other fish.
I have one of these in my African tank, and man is he aggressive. He's already killed an albino cichlid, and tried to kill another fish, but I added some more new fish and it dispersed his aggression. He's the boss of tank, but I also have a Turqouise Jewel Cichlid in the tank who is actually the dominant fish. The Cobalt pushes around all the others while the Jewel does its own thing. The Cobalt really, really wants to push the Jewel around, but the Jewel smacks him every time he tries. It's not for lack of effort. Very pretty fish, and very active, but because of the aggression I would not recommend this fish for a Cichlid beginner. Mine didn't claim a territory or cave, it claimed the whole 200 L tank.
I have a large blue Cobalt that mated with and orange zebra which is over twice as small as he is. The babies are either a pale whittish color or pale orange with some black spots on forehead. They are housed with 4 yellow labidochromis, 2 demasoni, about 10 of the juvenile Cobalt/orange offspring, and a pleco in a 280 L aquarium. All seem to get along with little aggression. Cobalt is the largest and dominant fish of the tank.