Sigh...I have seen this look before

[GreenSpottedPuffer]

Quite hard to see in the pics but here ya go. Its the darker area behind his pectoral fin. It almost looks like a bruise? Its not on the other side. My first thought was the liver (cyanide caught fish often die if liver failure) but the liver is lower and in front of the pectoral fin. Kidney could be a possibility but I don't think so either.