I'm moving my goldfish out of his 10gal into a 100gal, and plan on getting another goldfish as a tank mate. How long do I need to qt and fish for? And any other helpful hints on it?
3-4 weeks in the 10 gallon for a small goldfish should be fine assuming you do water changes frequently to avoid buildup of toxins. But it is the safest way of protecting your current fish and tank setup.
A cheap larger qt can be made using large but cheap rubbermaid containers. The filters will work fine on those as well.