When can you put goldfish into a tank of clean water?

This really depends on whether you have live rock or not. If you have opted to use live rock as a filter, then allow the tank to cycle fully before adding fish. This is generally 4 weeks, but you can confirm using test kits. If you are not using liverock, but are using a canister filter or other type of filter, then put a very hearty variety in serveral days (3 is a good number of days) after having the tank fully setup and running. The only fish that are likely to survive in the first 6 weeks are Damsel fish.