Jacks are NOT the devil, why someone unless they knew nothing about them would keep them with small community tetras, goldfish, dwarf frogs, and a dragon goby is beyond me(Dragon Goby really? its been said repeatedly and a simple google search will say straight up they need brackish). I keep 2 jacks and they do fine with the fish I've kept them with. Keep them with other American cichlids of similar temperment, or big dumm hardy fish like tin foil barbs, silver dollars and balas and they will eventually realize "oh, these are big dumb fish and pose no threat, I'll let them be". My experience, a jack will "test" a new fish, letting it know he runs the show, if that fish is to little or doesnt back down it may be killed. Given ample space and knowing your fish's personality is key. A 55 is only really suitable for 1 adult jack(mine was in a 55 by himself for about 2 yrs), However, I have kept him (an 8" male jack) with juvenile cichlids in a 55 successfully but this was short term, about 4 months (I was growing them out for my big tanks) he didnt kill any of the babies. He know lives with several of these growouts in the perm tank. Oh if it makes any difference to you, he is 3 yrs old. The point I'm trying to make is that every jack is different, don't expect a killer or a peace loving hippie jack, they are all different. Check my signature for fish that have worked for me, he has been with all of them except for tiger barbs, cories and the pleco at one point in time.