Sorry I've been out of touch for so long. Gah! I hate errands.
About the salt, since you are seeing jagged edge I would go ahead and add salt at 1 teaspoon (5 mL) per gallon. That will get a concentration of .1% which is what we use to treat bacterial infections. Goldfish can handle salt for a while so go ahead and add it. Just make sure you put an equal amount of salt back in when you take out water.
A bubble stone is a great idea. Goldfish like highly oxygenated water. They will also play in the bubbles. I like the Fusion air pumps as they are really quiet and last a long time. Rena pumps are also pretty good. The pumps are generally pretty silent, but the bubbles coming out of the water do make noise. I use bubble filters in my room, and there is more noise than you would think.
Goldfish and koi are very social creatures. They're not true schooling fish, but they love company and will be visibly sad and stressed if removed from the group. I recently had to separate my goldfish due to some nipping issues, both goldies spent all day swimming at the divider trying to touch each other. My koi are also very social and tend to follow the leaders.
Savannah brings up a great point: always quarantine new goldfish for at least a month to monitor for illness. I'm sorry you lost your goldies; sometimes we have to learn lessons the hard way. I also had a brush with ich due to a lack of quarantine. I also hope you now know that a 10 gal is too small to keep a goldfish in for very long. It's best to just divide it and get two bettas. ;)