My Danio is swimming in circles and listing to the side ~ whats wrong

It is basically close to death my friend! Here is a thing:

Water quality is usually the main problem when a fish's condition

changes abruptly. The next thing to do is make sure you have the highest water quality possible. This would mean 0 ppm of ammonia, nitrites, and nitrates; all of which you can test using a test kit or by having your local pet store test them for you. Don't add any more fish until you've had your water tested. If any of these levels are too high, you should do an immediate 25% water change. If your levels of ammonia are high, three 25% water changes over the course of a week may help.

In the future, you should perform a 25% water change every two weeks. This will help to control levels of ammonia, nitrites, and nitrates. The water changes will reduce any problems with water quality and help the fish combat parasites plus reduce any harmful bacteria.

In order to maintain low levels of these toxins, you should also make sure not to overfeed your fish or overstock your tank.

Overfeeding of your fish can increase these levels in the water as the uneaten food is allowed to decompose. At each feeding you should feed no more than what your fish can eat in ONE minute, while allowing a minimum amount of food to fall to the gravel. When it comes to feeding fish, less is more.

Overstocking the tank can increase these levels also. As a general rule of thumb you should have one gallon of water per inch of fish when stocking with tropical fish and three gallons of water per one inch of fish water when stocking with goldfish. Excess waste caused by overstocking, like excess food, gives off ammonia and nitrites.