Nitrate levels in my freshwater aquarium are high what do I do?

The nitrate levels in my freshwater aquarium are high. I'm not sure what I should do. I did a 20 % water change Saturday and then again yesterday. The levels are still high. The tank has been set up for over a month and I let the tank run for a week before adding fish. When I did the water changes I used 1/2 distilled water and 1/2 de chlorinated conditioned tap water. I also added stress zyme biological filtration booster in the most recent water change. It is a 10 gallon tank and I have 3 mollies and 2 ghost shrimp. I have been told Mollies are not right for a 10 gal aquarium. But when I purchased them, at Pet Smart, I was told they would do just fine in a 10 gallon aquarium. I probably shouldn't have listened to the salesperson but I'm new to this hobby and didn't know any better. Please help!!! What should I do now? I don't know if I can return the fish since I've had them for over a month. I cannot afford to buy a larger aquarium right now.