Can a Jack Dempsey and an Oscar Fish Live Together?
One of the banes of a any fish keeper's life (and perhaps, one of the joys too) is putting together a tank of fish that don't slaughter each other. Large cichlids, such as Jack Dempseys and Oscar fish are known for levels of aggression from mild to very aggressive. So can you keep them together? The answer is yes, providing a couple of conditions are met.
First of all, the tank must be large enough. A 55 gallon will only just cut it, and many would say that even that is not large enough, that a 90 gallon is needed. Space is important to fish, and being crowded only increases agression amongst already aggressive cichlids.
If you are going to keep aggressive cichlids together, make sure that they have plenty of room. Make sure too, that there are hiding places for both fish. If you have more cichlids in the tank, ensure that they too have hiding places. Cichlids like to be able to hide, so rock caves and other places to hide are an absolute must.
If you design your tank carefully, you can provide spaces for smaller fish to retreat when they are threatened by larger fish. As a rule of thumb, you should never put fish small enough to fit in an Oscar's mouth in the same tank as an Oscar if you are not prepared to lose them. And when I say lose them, I mean, see them killed by the Oscar. Oscar fish are carnivorous, and whilst they are happy to live eating pellets, which is their recommended diet, they will eat smaller fish if the smaller fish hangs around long enough to be eaten. Some red tail sharks can live with Oscars, because they tend to be zippy, predator aware fish who do well in amongst larger, more aggressive fish, but one day they may be cornered and consumed, so if you are particularly attached to them, remove them from the tank.
Returning to the subject of the Jack Dempsey and the Oscar Fish, JD's tend to be more retiring in general, so you can expect to see a great deal more of the Oscar than you do of the JD. This is entirely normal behavior, and is not a result of the Oscar 'scaring the JD'. Now, depending on size and temperament, a JD may try to bully an Oscar. Some are even successful, others not so much.
In my case, I have a larger Oscar fish and a JD about half the size. This has proven to be a fairly successful mix. Whilst the JD did try to bully the Oscar at first (the Oscar was a late addition to the tank), the Oscar quickly showed him who was boss. Younger, smaller Oscars may fall prey to bullying more easily.
The long and short of matters is that it largely depends on the temperament of the fish, the lay out of your tank, and most importantly, its size.