jewel cichlid with jack dempsey

Question
i have a jewel cichlid in an 18 inch tank.  it was real pale in colour when on his own.  i was advised that adding a jack dempsey would be fine, so did.  they were exactly the same size when added (2.5 inches or so).  the jewel has completely destroyed most of the jd's fins/tail in one day.  ive tried rearranging the tank so the jewel doesnt feel like hes the boss.  i dont know whether this will work.  looking on the web it seems ive probably made a mistake as the tanks a bit small.  any advice to help keep em both?  i also have a 2 foot tank with two convicts (3 inches or so breeding pair with fry)  could i chance putting one or the other in with them?  need to get the jd healthy again really?  any advice appreciated.

Answer
It would be good if you could give the Jack Dempsey his on tank, but if not you could try adding the Jewel to the other tank for a few days, this will give the JD time to get accainted with his new tank and to start healing, then when you add the Jewel back he won't feel like he is the boss anymore.