Ammonia, nitrites, and nitrates are all toxic to fish and must be quickly and effectively removed from the aquarium. A proper functioning biological filter will do this, and help establish a happy, healthy… 

…quality issues, generally indicated by high nitrate levels. In closed aquarium systems, impurities such as nitrate increase over time and cannot be efficiently removed. 

How to properly reduce nitrates and replace trace elements for healthier fish and live plants. 

There are three types of filtration for use in aquariums, chemical, mechanical, and biological. Of these three, mechanical and biological are a must for every aquarium.