Is there enough oxygen in fish tank?

To raise oxygen levels you could use an air pump connected to an air hose with an air stone at the other end placed inside the tank. Or you could place live aquatic plants in the tank.

For the most part, people control oxygen concentrations with aeration and limiting the amount of living things and feed you put into the tank. The more fish you put in, the more oxygen they will require. Also, be sure not to overfeed, since decomposing food will also require oxygen. Beyond that, aeration is the only practical way to control oxygen. You want an air diffuser that produces very small bubbles. The bigger the bubble, the less efficient the aeration (the bubbles contain air that pop at the surface, which means that air is NOT getting into the water!).