How can i reduce nitrate in my tropical fish aquarium?

Water changes are important as many here have mentioned. However based on the age of your aquarium and your nitrate levels water changes alone will only address the symptoms. You most likely have high levels of DOC (dissolved organic compounds), this organic mulm is often present under decor, in filters (especially canister filters that are not properly maintained and in UG filters). You need to look for these sources of organic mulm so as to better control your nitrates. Another way to control nitrates (this should not be used as the only way though) is through live plants. Many live plants, Hornwort in particular remove large amounts of nitrates from the water and many such as Java Ferns and Java Moss grow well in low light conditions. I would highly recommend reading this article for much more information about the nitrogen cycle including nitrate control: <a href="http://americanaquariumproducts.com/Nitrogen_Cycle.html" rel="nofollow">http://americanaquariumproducts.com/Nitr...  more