The only thing that will reduce nitrates is changing the water. Make sure before you put the fresh water in you De chlorinate it other wise the Chlorine will kill the good bacteria thats living in your filter and tank.
There is a Maidenhead Aquatics not too far from you. The bit on Stockton where all the car garages are near Preston Park, there is a big Garden Centre with a Maidenhead Aquatics there. if and when you want some media drop me a PM and we can arrange to meet there. Bring a bucket so I can puour water and media in and Ill take your new media and put it in my filter.