What's wrong with my guppy?

[AA1]

Hi Everyone,
Any suggestions as to what might be wrong with my guppy? He's stopped swimming with the other guppies and seems to have developed a strange shaped belly (see photo). He's still eating, but is generally staying out of the way towards the top of the tank (though he's not gasping at the surface).

240l tank with fluval external filter 406 filter
30 small fish (nothing larger than a guppy)
ammonia nil
nitrites nil
nitrates <25

Any thoughts greatly appreciated.

 

[SaraB]

He might be constipated. You can try feeding it a cooked/shelled/cut up pea to see if it helps.

 

[AA1]

Thanks - I'll try that and see if it helps