Help, black flecks on goldfish :(

Hi, one of my goldfish seems to be having a few problems. I noticed
yesterday that he had some black flecks on his side. I tested the water
for pH, ammonia, nitrate, nitrites, etc, and the levels were all good.

I did a half water change last night, and just checked him again...
there are more black flecks on his body, a few on his fins and he is
sitting on the bottom of the tank and breathing hard.

He is not clamping his fins, and is still swimming about when the mood
takes him, but he quickly returns to the bottom of the tank.

I'm pretty worried, can anyone tell me what's going on? The only things
I can find are about ammonia burns, and black spot disease.

Thanks, Jo