1. How big is your tank: hospital tank is 17 litres

2. For how long has the tank been set up: over a year

3. How many fish: Just 1, 'Magnum' that I have had about 3 months.

4. What kind of fish: Not sure, just goldfish

5. What are the water parameters:    Ammonia 0 ppm
                                                            Nitrite 0 ppm
                                                            Nitrate 0ppm
                                                            pH Unsure
                                                            Temp Unsure

6. How often do you perform water changes: At the moment, daily

7. How big are your waterchanges:     50%

8. What kind of water conditioner are you using: Tap Safe with Aloe and 'good' bacteria

9. Do you add beneficial bacteria: See above

10. Symptoms your fish is showing:

About a 10 days ago, shortly after I added a new plant, he started acting a bit odd, biting the bubbles at the top of the tank.
As the week progressed he started sitting at the bottom of the filter so I started watching him closely.
He then became lethargic and laying at the bottom of the tank, not moving. I noticed that he had quite a bit of rotting food at the bottom of the tank, I decided to do a 50-60% water change. I added 'Goldfish Tonic' as I didn't know what was wrong with him.
He then got white cotton-like orb things attached to his sides *see photos
They then started coming off of him once I did a 50% water change and added some 'General Tonic' and 'StressZyme' to his water.

I went and got Aquarium Salt and Methalyne Blue and added those the tank. For 2-3 days he started improving so put him in the 50 litre main tank. He deteriorated fast. Most of his tail is gone now, and it appears the orbs on his sides are coming back. He also has stringy bits hanging from him - I thought it was just poo but could it be a parasite?

So, another 50% water change in the hospital tank with Methyline Blue, Goldfish Tonic, Salt and a few food flakes and 2 sinking pellets.

He isn't moving much and looks so dreadful. I am worried he won't make it through the night. 

I have switched off the air pump and carbon filter completely - am I right in doing so?

I spoke about this on another forum and was told to put him down. This horrified me.  

I have added some photos.

Any help and advice would be greatly appreciated.

Ron, nevermind credits here! If you remember those things, that's your knowledge now

In any case, I feel really sorry for the fish But if I understood everything correctly, you moved him far too early! You need to keep the fish separate and treat according to instructions until all the visible symptoms are gone!!
He really doesn't look healthy, and there's quite a lot of that fungus going on there... Are you saying that you're changing 50% of the water in the quarantine tank every day?? If so, the medicine can't work! You have to be patient about it.

And the test results are pretty strange, there's no way there would be 0 on every parameter. (Not to mention that you don't even know the temperature of the water, which is a lot easier to find out.) So, please buy yourself a proper test kit! Check the parameters above every day when treating the fish.
Are you vacuuming the gravel when you change the water too??

I'm afraid there's not much you can do for him anymore, you were close at some point but then you rushed... :/ 2-3 days isn't nearly enough time to get a fish healthy, remember that for the future.

Poor fish!  I'm sorry he is so ill, QueenG. It looks like a super bad case of fungus and fin rot. When they start laying on one side with labored breathing, it can be hard to pull them back from the brink. As horrifying as it seems, my advice would be to put him down. The fish is obviously suffering and is not getting any better after quite a bit - it would be kinder to put him out of his misery when he has such a low chance of survival. Take it from me, it's awful having to put down a pet, I've had to do it before when fish of mine contract dropsy I can't cure and it's very sad. But I think it would be better for you to put him down now than to watch him die a slow and painful death. I really am very, very sorry he came down with this horrible disease. Please don't think I'm being unsympathetic or harsh. That's just what I would do in your place.

I was just about to prepare to euthanise Magnum when I discovered that he had passed away. 

I am very sad that he suffered so much.

Thank you for your advice everyone.

That's so sad
But it would probably have been hard to do it yourself anyway, I mean, when it's about your own fish it's usually that little lingering feeling that... "He could still make it...?" and that sort of slows it all down...

I know how that feels, QueenG. When I had my very first goldfish with nitrite poisoning I didn't know what was wrong with her...I was only a beginner. I knew in my gut she wouldn't make it, but I thought to myself, "she could still pull through maybe!" It really is a hard thing dealing with a fish who's dying.