Good afternoon all
I appreciate that it is now many years since the last thread was posted on this site but I've been researching the same problem.
I too have a comet (I think) now probably 8 to 10 years old which spends most of its time lying on its side at the bottom of my pond. It comes to the surface to feed but clearly has a buoyancy problem as it has to work to stay there.
It looks to be in good condition - the main difference is that mine has been behaving like this for about 3 or 4 years!
Its pond mates are fine so I'm not inclined to do anything about it, I just thought I'd throw this in for possible interest.