When I grow my Dempseys out in large tanks, they grow quickly...
When I grow my Dempseys out in small tanks, they grow slowly...
This is why many people believe that Dempseys grow slow, because they slow their growth with small tanks... I'm not saying this is cruel or wrong or anything like that, just sharing facts I've learned through experience...
I feel that a 9"+ Dempsey needs to be in a 48" x 18" footprint or bigger, to prevent him from having issues of a bent spine. I also understand not every Dempsey gets that large, especially females. Smaller growing Dempseys usually will not get a bent spine in a 55 gal, but larger growing ones often do.
I definitely would not add any more Cichlids to your 38 gal tank. But feel free to add whatever smaller non-aggressive "community fish" such as Tetras and the like. In time, the Dempseys will grow large enough to eat them, but by that time you will have already needed to upgrade them from the 38 gal.
Also keep in mind that as adults, Dempseys are solitary fish or pairing fish. meaning they do not want to hang around their own kind, unless they are a breeding pair. So if you have two males they will almost certainly fight for dominancy and the tougher of the two will suppress/stress the other. Two females are less likely to do this to the extreme, but are rarely "friends" either. If you have a male female then they will most likely pair off and make you many many babies.
