Try adding aquarium salt (or non-iodized salt) first. The salt will help bring up electrolytes.

What's the water temp?

Water temp set at 75 degree, I just upped it to 80. THe fish is now arched over sideways, in that familiar dropsy pose. I had 2 others die of dropsy in the last year.

Is it too late for him? I won't be able to get salt in the tank until after 6PM tonight, it is now 7:15AM....

80 is way to warm for goldfish. They like cooler temps and will be okay in the low 70's. The main thing is that the temp stays stable.

Lisa

Fancy goldfish like warmer temps. If these are plain goldfish/comet type they'll prefer cooler temps.

Unfortunately, dropsy is something that isn't always curable.

Also another thing, if it's a digestive issue - try feeding green peas. I use frozen organic, defrosted, peeled. Sometime peas can clear out various obstructions which cause gas which may cause the fish to swim strangely. It also helps to clear out other digestive upsets. To encourage feeding, you have to withhold feeding regular foods and feed the peas. All of the carp (goldfish, koi, shubunkin, comet) that I've had pretty much eat anything floating down through the water. How's the sick one's appetite?

I cranked the heat up when I saw something was wrong. Warm water is good for salt treatments. I have heard about the pea thing, will try it but he is too far gone to even get to the food, i believe.

What is the history? You cleaned the tank but what about fish? Any new additions to the tank? How old are the fish? How old are your test kits?

Fancy goldfish do indeed need a bit warmer water.

Epsom salts will help flush the fish like peas, pepso food and other remedies. The nice thing is if the fish has stopped eatting it will still get flushed. 80F should be OK for a while. It helpss speed up metabolism, keep an eye for aggression in the others though. You may have pulled up alot of anerobic bacteria "clumps" and the fish ate it. Maybe a reason. If you have a QT tank it may be a good idea to isolate the fish until you can ID the problem or "cure" it.

I came home and he was dead on the bottom. I am not certian it was dropsy, as he did not have the pine cone effect. He was arced sideways. Poor thing. I feel bad that my petsitter did not see it.
I have salt now and the 3 remaining fish look fine. Should I add some just to be safe?

Under gravel filters (UGF)and bacterial problems seem to go hand in hand. I would remove the UGF. They seem to be the cause of bacterial blooms causing cloudy water and could possibly be the culprit???? Salt will help any stress at this point, thats about it. Other "salts" beside sodium chloride that are helpful in the aquarium are epsom salts and potassium chloride.

a little salt won't hurt, and quite honestly, I always add pond salt to my goldfish (fancy) tank after water changes.

This info is found at the link below:

Aquarium salt is widely used to help prevent illness as well as treating for different diseases. Salts are widely used because they have so many benefits compared to using medications.

Examples:

Salt will not harm your filters
It is cheap to use
It does not cause any harm to the person administering it
It is not harmful to the majority of fish species
It will not send a sick fish over the edge
It quickly eliminates 7 out of 9 parasites
Before using salt you will want to remove submerged plants and perform a water change of 50%.

Salts that can be used are:

food grade
non mineralized cattle feed grade
NON-iodized salt (no additives such as Yellow Prussiate of Soda)
Aquarium salts available at the petstores

Below is a helpful chart to use when help is needed!

However, the recommended dosage from Doc Johnson is 1 teaspoon of salt per gallon of water every 12 hours not to exceed 3 treatments. If dealing with large tanks or ponds you will need to use 3 pounds of salt per 100 gallons of water.

Also read the information below:

Salt levels do not dissipate with water evaporation or small water changes. Freshwater/coldwater fish like Goldfish are not designed to live in a saline environment. Salt is a wonderful remedy to treat ailments, or to control the toxicity of nitrite/nitrate spikes, but as a normal maintenance procedure it is a no-no for goldfish. Salt also tends to make water harder. If you already have hard water, you are adding to his stress. Through what is called "osmosis" your goldfish already maintains a normal high salt concentration in his body fluids and needs to absorb freshwater into his cells to keep in balance. By adding more salt to his water, you are overloading his system and putting him under osmotic stress. This can be fatal. It saps their energy and strength and weakens their immune system, or can even kill them. This is probably why your fish is spending a lot of time "sleeping" at the bottom of the tank. Give him some fresh oxygenated water w/o the salt. Save the salt for only when he is sick or there is a problem with your water. Happy fishing!!

Salt Dips-

Providing salt dips help fish that have been shipped or moved recover their electrolytes.
A fish that has been dipped in salt is more susceptible to medications being able to help them since the salt dipping will remove their protective covering (slime coat).
Dipping removed unwanted parasites

Low Salt Dosages-

Low salt dosages in the tank will provide minerals for the fish.
Works like a protective covering on the fish to protect against nitrite poisoning.
Will stimulate the slime coat

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There's a table at that site, too - that reflects dosage and use.

And this link:

http://www.koivet.com/html/articles/articles_results.php?article_id=149&category=17&search_term=peas

will explain my reference to feeding peas.

Here is a link that might be useful: Salt info