Help! Dying Fish!

I'm a complete newbie to this site and goldfish keeping in general. I'm looking after my dad's place while he's away and I've noticed the goldfish in the pond aren't doing too well. One guy in particular is in a bad way. He wasn't moving much and sinking to the bottom with white stuff coming out from his gills as well as a white residue on his body. I took him to an aquarium and the guy said he didn't know what it was and didn't offer any advice as the fish was probably going to die away. Needless to say I though he was an idiot and started searching the net for answers.

So far I've brought him inside and isolated him in a "hospital" tank which has a filter and airator. I put some water conditioning drops in and checked the PH which looks around 7. I also gave him a salt bath of 30% for a minute or so until he rolled over (I thought I'd killed him) then put him back in the hospital and some fungucide drops in the water. He's picked up a bit and is now swimming at the surface again but not strongly.

Can anyone suggest what else I can do? Should I feed him or not? If so what should I feed him. I read about frequent water changing of 30%, is that just with straight tap water? I thought changes to the water would shock him. Like I said I don't know anything about keeping goldfish and I don't want the little guy to die on my watch.

Any advice would be gratefully received.

Cheers,
KMJ