Hi there! I'm new here, but I've been keeping fancy goldfish for about 5 years now. I usually only add salt when quarantining new fish or treating sick fish. I use regular aquarium salt, and start salting at about 0.1%, and work my way up slowly to 0.3%. I keep it at 0.3% (with prazi pro) for about a month when quarantining. I do not add salt to my main tank now unless something shows up in all my fish I need to treat (doesn't happen all that often). IMO, they don't really need it or benefit from it when healthy.
(For reference, 0.1% of 10 gallons is about 37-38 millilitres, and 0.3% is around 113. Since aquarium salt doesn't dissolve, I only replace a small bit at each water change, not a full dose). I like to dissolve the salt in tank water separately before hand, so my goldies don't eat it.
Hope this was helpful! Good luck with the fancy goldfish tank!
