Conditioning Your Tank for the First Time - Chemicals to Alter Your Aquarium Water

There is only one water conditioner you need, and that's a dechlorinator.
via firsttankguide.net

If you use tap water, it's imperative that you use a water conditioner to break down those harmful metals that could potentially kill your goldfish.

In my first days of fish keeping over 10 years ago, I would put my poor goldfish in a tank filled with unconditioned tap water, only to find my fish dead the very next day.

Don't make the same mistake! Keep that water conditioned. I've been personally using AquaSafe for years, and it works very well. Plus, it's cheap and you can pretty much find AquaSafe in any pet store you frequent.