Updated: May 20, 2021
Water quality is very important to proper function of an autoclave. Tap water can lead to clogged valves, clogged internal lines and even stain your instruments. You should only use distilled water in your autoclave or De-ionized water with a TDS (Total Dissolved Solids) level of 5 or below (at or near 0 is ideal).
The Statim cassette sterilizers (2000 & 5000) even have built-in water quality sensors to help protect these sterilizers from poor water quality. They will fail to function if the water quality sensor detects a TDS level above 5.
As mentioned, in addition to distilled, you can use de-ionized water. De-ionization is basically super filtration of the water to remove minerals and particles. A common place to find de-ionization equipment is your local tropical fish store. De-ionization has been popular for treating aquarium water for a very long time. If using a de-ionizer, be sure to also get a TDS meter (these are widely available and usually for <$20) so you can be assured you have proper water quality.
I do not recommend keeping fish in your autoclave, however.