What temp do you keep your tank? The water should be around 26 degrees c for tropicals depending on the fish If it's higher then the water can't hold as much oxygen. To add more oxygen, either add an airstone to pump air through the water, or position your filter so the return breaks the surface of the water and makes bubbles in it.
I would also get a test kit that tests for Ammonia, Nitrite, Nitrate and pH. These are the basic measures of water quality.
How often do you do water changes, gravel vac etc?