Welcome to APC, Wendy
Is the water dirty because the fish is stirring up the substrate or because the tank has not been taken care of?
Has the tank been cycled?
Is the tank planted?
What kinds of fish do you have?
Have you tested the water parameters?
Give as much info about the setup as you can, so that we will be able to help you better.
With out knowing all the answers to the above questions, I would say that you at least need to do a large water change (75%) to clear up the water.