I keep Comets and some common Goldfish, so I understand them a little. Fish problems are more times than not related to water conditions. What is your water change routine? You know Goldfish are heavy waste producers and need their water changed at least weekly and you need to remove and replace most of it to keep the dissolved waste out of the tank. This may be stress related due to higher than normal levels of nitrogen from dissolved waste material.
I wouldn't put chemicals into the tank. This is risky. Gradually work up to the point you're changing out most of the tank water every few days and add a bit of standard aquarium salt to the new, treated tap water. I use a healthy teaspoon in every 5 gallons of new water in my Comet tanks. This will boost the fishes' immune system and pathogens and parasites won't tolerate pure water conditions with a trace of salt.
"Fear not, my young apprentice. Just change the tank water."