We bought our shubunkin Rosie in 2004 and for her first few years she lived in a 4' aquarium. In 2010 Rosie and the two boys she lived with spawned and we re-homed them and their babies to an indoor pond which I now maintain.
A week or so ago someone at the pond buildings told me that Rosie was looking unwell. When i arrived she was floating around on her side so I brought her home and housed her in a 145l storage tub as this was the biggest thing I had. For the first few days I had to hold her and let food drift right in front of her mouth.
I have an established tank for my axolotls so borrowed half of their filter sponge. She has a Fluval 2 plus in the box and eats quite enthusiastically (blood worm and shelled pea) but otherwise she just sits on the bottom. I've treated the water with aquarium salts but I'm unsure what else I could try. I'm half tempted to ring my vet to see if they can help. We've taken goldfish to them before but they weren't in as bad condition. I might ring them tomorrow to see if they'd give me anything without me having to bring her in as I think she'd really struggle with a car journey.
She does look really skinny and I'm pretty sure she's totally blind but we've had blind goldies before that coped OK.
I bought some melafix but that seemed to make her worse if anything (she started lying on her side rather than upright). We changed the water about 24 hours after adding the melafix.
Just as a reference Rosie is on the far right of this photo which I took in August. Their pond is around 1200 gallons with around 15-20 goldfish in it. It has a big laguna filter and is fed by the a mains tap which runs through a carbon filter.
This is a difficult one as you're doing all that I would recommend (ie putting her into a QT with established filter media, trying bloodworm and shelled pea in the event that an internal 'blockage' was leading to her floating, aquarium salts). I'm assuming that water quality is fine (ie ammonia and nitrite at 0, nitrates low) and hasn't somehow unbeknowingly deteriorated?
It's possible that a broad-spectrum medication for bacterial infection (eg Waterlife Myxazin, eSHa 2000) might help but, with that reaction to Melafix, I'd be concerned she might have a similar/worse reaction to another medication - although perhaps giving it a try at half dosage is worth a go. I agree that, if your vet has expertise in dealing with fish, it's worth calling them for their views; let us know what they advise.
Water parameters at the pond are ok but we're finding them harder to maintain in the tub. We wondered if perhaps Rosie was depressed at being on her own so brought a smaller fish home from the pond to keep her company. We also bought a smaller size sinking pellet which she loves (they are fed floating pellets at the pond). We're not sure what's caused it but Rosie seems to be a lot better and is swimming around now.