You haven’t described what exactly is the issue you are facing with your goldfish. Is it panting too much (as in gasping) or has it kind of settled itself down and not moving much. Check if the air supply is functioning well and turn it up as much as you can. Secondly, do a water change, at least 50%, but ensure that the water is de-chlorinated first and is at the same temperature as the aquarium water. You can still use the old filter in parallel, as long as the media in it is still wet, as the bacteria will still survive partially in those conditions, but I wouldn’t recommend that now. Water changes will remove any ammonia traces, till the new filter settles. Add a de-nitrifying bacteria culture if available.