Another cheap, easy way to improve your levels is with an airstone/air pump.
Aeration/Oxygen: Your fish and bacteria have to have oxygen and other gases circulating in their water to survive. There are a few ways to achieve this. Filters with higher flow rates agitate the water surface more, which can improve gas levels (just make sure your fish/plants can tolerate higher flow. Fish with long fins and plants with fine leaves sometimes don't). Another cheap, easy way to improve your levels is with an airstone/air pump. We swear by them.