Hi Alan and F15hguy,
Thank you for your responses. I have 9 fairly large fish (comets, blue fantail, calico fantail, white fantail, Lionshead, oranda shubunkn) in a 700L pond in CHCH. I have never had an issue with water quality (over last 2 years) but I have to admit that I stopped checking as it has always been perfect (I do a water change fairly often and have an excellent filtration system). My bluey is the only one who has been affected so I don't think it is related to poor water quality but I will get the water tested tomorrow and let you know the parameters.
F15hguy - I treated bluey in a quarantine tank so melafix hasn't been near the main filter.
Again, thanks for your time, I'll post tomorrow the water paras and will treat the whole tank with wunder tonic.
Cheers!