New Shubunkin fish

Question
Just today we installed two goldfish and two shubunkins into our 3m x 1.5m outdoor pond. One of the shubunkins appears to have an issue with it's balance - i.e: swims slightly on it's side, but doesn't have an issue with buoyancy. It also seems to sit in the corner of the pond more than the other fish. I may be over-reacting, but I really don't want this fish to die.
I did look into swim-bladder disease, but I don't think it is that. However, one of the websites I looked at mentioned that it could be another disease.
I would really appreciate it if you could let me know what you think, and whether we need to check this fish out.
Thankyou so much for your time.

Answer
The fish could have internal parasites or some other injury not obvious to the eye.
Your best bet is to isolate him and treat him with some fish medication containing "malachite green" as well as feed medicated fish food.
I am sorry to say, it may be too late to save him, though.