it is one of my very large st's.20"+..there seems to be a lot of hormones going around the tank..a large pair of zullies are courting, a large w/c Xingu is bullying everyone and now this.there are more st's in the tank and they are all eating a lot as usual and pretty much acting normally.
It could be fat storage. I notice some of my datnoid, as small as 10", develop it now and then. Usually those that eat like there's no tomorrow. Cut back on feeding and it will disappear