This can be normal depending on mood although water quality being bad will make them turn black.
check you water then let us know.
My gsps belly turns black when he sees food or is feeding when he's excited then it goes white again.
How can you say that, unless you actually know the OP personally?
To the OP: are you keeping this guy in Freshwater or Brackish water? Coz GSP's are known to require high-end brackish water to almost full SW conditions as they grow older... also, while puffers can change colour to a certain extent due to moods, they also tend to get dark like this when they are stressed out, and if extremely stressed, they can sometimes turn almost black...
any info at all would be appreciated
