How often should you do a water change? 

With goldfish it is highly important to do frequent water changes especially if you have a highly stocked tank. Goldfish are one of the most waste producing coldwater fish and if frequent water changes aren't carried out then the water can quickly become toxic to your goldfish.

How much water should you change? 

With goldfish a water change of up to 80% is advised, depending on your water quality determines how frequent these water changes should be, if you have high levels of Nitrite on you water (for information click here) 80% every two-three days is advised (when doing these water changes DO NOT clean your filter every time as this will kill your beneficial bacteria. If you have a high water quality then water changes of 50%-80% two twice a week is advised, when doing a water change don't forget to add a delcorinater to kill all of the chemicals in the water that are toxic to fish.

Won't frequent water changes kill my beneficial bacteria?

The simple answer to this is with care, no. Beneficial bacteria colonise on surfaces such as your filter media, doing frequent water changes shouldn't greatly affect your beneficial bacteria as long as you don't clean your filter with tap water, when cleaning a filter you should always use aquarium water because tap water kills beneficial bacteria. 

Will frequent water changes stress my fish?

Frequent water changes shouldn't stress your fish much as long as you make sure the water you are filling up your aquarium with is as close to the water temperature that is currently in your aquarium.