Sick fish: advice?

We noticed today that one of our goldfish in our outdoor pond is
looking lethargic and not schooling with the other fish.

Her abdomen looks enlarged but there is no "pineconing" of the fins.
She is hanging in the water with her head pointed partly downward, and
swimming weakly. There is a red round spot about 1/4 inch in diameter
where I think her anal port ought to be--underneath her tail fin and
sort of in front of the small fin below her tail fin. I've sees what
I think is stringy looking poop coming out from there and also a
little thin white stuff.

She's not interested in eating and hangs around in clumps of string
algae. Her gills look red, and I don't see anything that looks
abnormal on the rest of her body.

The other fish look fine, and I tested the water. The pH is slightly
over 8.0 and the carbonate hardness is good, negative for ammonia,
nitrite, and nitrate.

We isolated her in a 30 gallon tank with an air pump and are gradually
bringing up the salinity to around 0.2 ppm for tonight, then perhaps
to 0.3 ppm tomorrow.

Could she be egg bound? Is this dropsy? Columnaris? Something else?
Shall I gradually bring up the temp of her water? (It's been in the
low 60's).

It's Saturday night and I won't be able to get in touch with a vet
till Monday, and even then I'm not sure he's going to know any more
than you guys know--maybe less.

Any first aid advice you all can give me would be much appreciated.

Thanks.

Joan

P.S. I can email pix by private email if you think it would help you
figure out how to help me help her.