Im sorry fish arent my forte but i hope i can help as much as i can.
Firstly how long have you had your fish because something couldnt suddenly go wrong with the water could it- and if it is it's highly unlikely. Has he ever acted like this before when you previously changed the water.
Maybe you could try an oxidising pump to increase the amount of oxygen in the water to help him on his way.
This is a really tricky situation- perhaps its usual for this species of fish- perhaps they have wierd mating rituals, or hes reaching a different stage in his life. Possibly he has a cold or something- although im not sure whether fish can catch colds :S.
Im sorry i couldnt be of much help- i tried

I hope he gets well soon

Love Vicky
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