Hi BrooklynAngel,
I am sorry for the grim news but a 6g really is not a suitable tank size for the fish. Do you know what strain of goldfish do you have? If in doubt, please post a picture of it. In your second reply, by "them", do you mean you have more than one goldfish in there?
Fancy goldfish need to be kept in at least 15g guideline per fish. As these are sociable by nature, obviously, keeping one by itself in a 15g or at least less than a 40g is not really an option. They need plenty of space and the amount of space given, paves enough time to dilute their wastes as well thus preventing ammonia and nitrite for soaring and, harming and killing them subsequently. Please post your water parameters as we really need it. What are your nitrite and nitrate? Why did you add salt in your tank?
The likes of ryukins and pearlscales are quite prone to digestive upsets and swim bladder disorders as their body shape has compress their internal organs greatly thus enough air can easily be trapped in their organs and leave them to lose their buoyancy in the process. Please presoak the pellets, whether floating or sinking before feeding them and avoid the flakes automatically. They can ingest air just by eating floating foods or hog all the foods that the foods literally expand inside their organs thus they suffer the consequences of swimming imbalance.