You can add aquarium salt to the tune of 2 1/2 level teaspoons per us gallon. At first, you'll want to bring them to this level slowly. The best way to do this is to get the ammonia to 0ppm or less and the nitrites to .25 ppm or less, then you can start adding salt. You can continue to add salt until the tests register 0 for nitrites (up to 30 days with tank temp in the upper 70's). The initial addition of salt should be split up into three additions if water changes are more than 50% a day. If they are somewhat minimal, you can begin adding the 2 1/2 teaspoons per gallon to the gallonage that you're adding back to the tank. Basically, you are trying to make the salinity rise somewhat slowly so as not to stress your goldfish even further.
Freshwater Aquariums: Successful Goldfish Keeping ..
The importance of maintaining your aquarium on a regular basis cannot be stressed too much. This is a key element in keeping your goldfish healthy and happy.
How to Set up a Healthy Goldfish Aquarium: 13 Steps
When setting up your aquarium you need to keep in mind the requirements of your fish. Large, smooth gravel is most preferred since goldfish will sift through the gravel all day long and you don't want them swallowing rocks or cutting their mouths. Live plants aren't usually kept with goldfish because they absolutely love to munch on them. Plastic plants are most easily maintained in a goldfish aquarium. Decorations aren't needed in a goldfish aquarium, goldfish decorate the aquarium with themselves! But if you feel you need an eye-catcher in your tank, keep in mind the dangers of them. Delicate goldfish such as Bubble-Eyes or Telescope-Eyes should not be kept in an aquarium with sharp rocks. There shouldn't be any decorations with holes or caves smaller than the goldfish can swim through without scraping their sides and/or stomachs.
How to Set up a Healthy Goldfish Aquarium