Something Wrong With A Goldfish

[BlueShinobu]

theres no spot for another filter, would just buying a more powerfull one help?

 

[WILDER]

Yes as long as you move your old filter sponges into the new filter, or squeeze as much of the benifcial bacteria onto the new wet sponges.
Your filter not coping with the waste.

 

[Tokis-Phoenix]

Yes i agree with wilder that the tank does need stronger filtration and more regular water changes, even with goldfish tanks that aren't overstocked, goldfish tanks generally require more filtration than standard tropical fish tanks etc.
Remember that the salt should only be a short term thing to help the fish our while you are sorting your water quality out, when the water quality is good again (ideally always 0 ammonia, 0 nitrites and nitrates between 1 and 40) you should stop the salt treatment as it is not good in the long term for freshwater fishes health.
With getting better filtration, you can either have 2+ filters in the tank at the same time (your original one and a new one) or just a completely new much larger one (which you should clone the beneficial filter bacteria from your old filter into), it doesn't really matter that much how many filters you have in the tank as long as they can easily cope with the bioload and waste of the tank .