Thanks for all the help! - looks like my tank is overstocked, the link above (which was really helpful) says that it is at 172%!
The chemistry all seems to be good though - but think I might get another tank and take the load off a bit.
Looking into treating swim bladder as it seems treatable as opposed to whirring disease.
One of the things I have read that may help is adding some salt to the water - would that be ok? How would it affect the other fish and plants - any idea how much I should dose?
This thread has some discussion and a link on adding salt. Fancy Guppies Help?