...jags are part of the big guapote family and they get around 14 inches in length with a good deal of body mass. these fishes are on a new level of agression, size, and specifically jaw structure. they are pure predators and probably will eat your convicts or jewels if given a chance. they will also kill both of your oscars and probably your 3 jack dempseys will kill each other. ill guarantee you that at least half those fishes will die if kept in the same tank. 75 gallons was meant to hold no more than 2 oscars,maybe a pleco, and lesser cichlids or dithers. even if you take the jag out you still have 3 jack demspeys that grow 10 inches respectively. the rule of thumb inch per gallon doesnt apply with cichlids because of the territory they demand. if you have a pair within your 4 convicts then youre just asking for drama since they will probably take out the other 2 convicts and maybe the jewels. at the size they look, they probably can take out all those fishes in the tank. i also dont think that adding 12 agressive fish together makes them less agressive. the only way i see that happening is if they cant swim and end up lying on their sides. not to mention the waste that these fish produce, youll probably be doing water changes every day. im against it, sorry.