so i picked up a new jag today(about 5 inches). since from what i read jags are fairly aggressive i decided to put it into my jacks tank to add a variety and colour. low and behold it only took under an hour before my male jack dempsey attacked the poor thing and had it literaly quivering in a verticle position against the back of the tank. i know people mix jacks with all kinds of fish and yet i have had no luck with anything but the female jack. maybe he really is psychotic