I have been useing carbon for 20 plus years.Also use salt.
What size goldfish tank do you have? It is soon going to be hard on goldfish cause now that the weather is getting warmer quality of the goldfish is going to be weak.

First of all, searches lately show horrific results and lots of misinformation (me read since childhood at libriary, background in pre vet/ocean sciences and 45 years experience and research.)

Gold fish are not at all like tropicals - different digestive/metabolic system. They or any fish unless they are brackish do not need salt but can take some to add replace minerals (gets then into what type).

The older Dr. Innes books from 30's 50's as well used Epsom (magnesium) when they were constipated.

Re: Carbon or any other issue as well.

There is never one thing for all... it depends on one's bio load (goldfish should have 20 gallons per small fish and room to grow - organs are stunted (inner) when they are not even though on the outside they look "small" due to too small and environment.

I don't use carbon any more but then I have heavily planted tanks, over filter (bio bugs) change one filter at a time and understock.

Carbon removes organic matter but one must change it often or it gets loaded up then dumps these toxins back into the water.

The load buildup depends again on your bio load.

Salt - why not save it for when necessary - when fish need salt treatment. (gradual up and gradual down)

Need to contact me feel free. (have tons of great hard to find links from professionals, breeders so on)... wetwebmedia is one great link.

As well goldfish need plant matter in their diet - so tons of easy plants like anacharis is good for them and inexpensive for you.

Best Sherry

Some read information and post what they read. Some have done and then post what they have done.