I'm babysitting my friends' fish for them over fall break and one of them has a gold fish. It's only in a two gallon tank

. In a tank that small and with a
goldfish, how often am I going to need to do water changes? Also, the water here has to be treated before i can put it in the tank, can I use the same water conditioners I use for my tropical fish? The bottle says it is safe for
freshwater fish (and saltwater but I don't have to wory about that). She forgot to leave me the water conditioner she uses and her roomate is gone too and the door is locked.
Thanks!