Question
I am a proud owner of a goldfish. This is my first time owning one.. Can I use distilled water instead of tap water?

Answer
Goldfish or most any fish will not survive in distilled water alone as it's devoid of electrolytes.  You can add aquarium or marine salts to the distilled water and then use it.  Or, better yet, use spring water.  The only reason not to use your tap water is if it's polluted or the water chemistry is way out of wack (too low or high pH, etc.)

Robyn
fishpondinfo.com