Answer

Thanks for the clarification.

What's happening in your tank is called New Tank Syndrome. All new tanks must pass through something called the cycling process, see links below.

When a tank is newly set up with fish, the fish's waste begins to form ammonia. At this time there is nothing to consume the ammonia so its level
rises. Ammonia is harmful and sometimes lethal to fish as you've seen.

After about 10-15 days a bacteria called nitrite
consume the ammonia and it disappears. Nitrites are also harmful and lethal to fish.

After about another 10-15 days a bacteria called nitrates consume the nitrites.
Nitrates are only harmful in high amounts and can be lowered/eliminated by partial water changes.

Once ammonia and nitrites are undetectable on a test kit the tank is considered cycled.

So what is basically happening is that your fish is suffering from the ill-effects of ammonia and/or nitrites. The way to fix this is by doing an immediate partial water change.

Aquarium salt is helpful but it should be used in a ratio of 1 teaspoon of aquarium salt per every 5 gallons of water as the salt will help the fish's gill functions and overall health. Good aeration is also a must and do not use a heater in goldfish tanks.

At tis point in time it's very important that the water be tested for levels of ammonia and nitrites. If the levels become too high then a 25%partial water change would be in order.

Keep in mind that if ammonia and/or nitrites are present no medicine in the world will help a fish.
Note that both ammonia and nitrites lower a fish's immune system thus making it more susceptible to disease.

So good luck and please let me know if you have any questions.

http://www.thetropicaltank.co.uk/cycling.htm

http://www.bestfish.com/breakin.html

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