Jack Dempseys are one of the medium sized aggresive cichlids that do need lot of space. If you have one make sure you got extra space for it from the other fish or it will fight for terrotory. I have two Jacks. One is 6 in and the other is 5 in. They get alone well in their 30 gallon by themselves. I've lost a lot of fish to these guys. They killed my convicts, red devil, midas cichlid. I'll soon be moving them to a 55 gallon which will give them plety of space. Telling the sex of the Jacks is easy compared to other fish. Males get up to 8-10 inches in length and females are some what a bit smaller. Mine are both male because their fins are pionted and they are much more sparkly. For breeding you'll need a hiding place for the parents to lay their eggs. Once laid they will protect them with all their will. When i had a breeding pair they would bite my fingers with their tiny teeth when i was trying to remove the eggs into a breeder tank. If you want to have other fish with Jacks you should have larger cichlids or ones that are just as tough as Jacks and that can take care of themselfs.
I wanted to post this comment about my two Jack Dempseys. In a precious comment, the poster stated that putting a type of Danio in the tank would be safe. My experiences with trying to have anything else in my tank BUT Jack's has failed miserably. I've tried Betta Fish, Danios, Barbs, Tetras, guppies, Cory's and goldfish. The Jack's killed and ate each fish I tried to put in there. I was attempting a community fish tank and it didn't work. They thought those fish were treats. I wasn't disappointed at all, I knew what I was getting into and I knew the dangers of putting anything in the tank with those two blood thirsty killers. I have named them Butch Cassidy and the Sundance kid. They're from the same tank from the Pet store and they've been together for the past year. Great fish. Just don't put any fish in there with them.
I have had a dempsey since has was 3 cm. When he was 8 cm long he chased and killed my 2 oscars (30 cm), had fights with my jaguar cichlid (20 cm) and always won, and dominated my breeding pair of 10 cm convicts. When he was 10 cm he bit my finger when I was gravel cleaning and made it bleed. This fish is definatly the most confident, dominant and aggressive fish I have ever had. I have now bought him a female as a partner. They have set up a small territory in the corner of my tank and are generally leaving the other residents alone. They have spawned and he seems to be happy now, they have made freinds with my convicts and are happy to mix and look after each others fry, they even sometimes swim together as a four. The big jaguar cichlid minds his own business. All of these incidences happened in a 300 L aquarium. The dempsey pair are now both 13 cm. A very fulfilled fish with lots of personality!
I started out with one jack, he was about 3 cm when I got him. I started him in a 20 liter. By the time he got to about 6 cm I moved him to a 110 liter. Now he is in a 220 liter tank with three other jacks and two african cichlids. Now my first dempsey is about 15 cm, a little over a year old. My big dempsey seems to be pretty shy and he's not very aggressive, but when he comes out of hiding the other fish make room. He barely ever comes out unless it is dark. I'm pretty sure he is a male beacause he's pretty long and thin, he also has no metallic spots on the bottom of his gill plate.
These guys are very agressive and are NOT a community fish. This fish is not afraid of anything and if it wants to kill something, it will. I had one that beat up every fish including pikes, oscars, green terrors, and other Jacks. If you keep one with other Central American cichlids that can fight back, this is a great fish to have.
I have a jack dempsey which has mated with a green terror. I have hundreds of tiny babies. I am very interested to see what they will look like when they grow. Both fish are very protective towards their young.