Still water changes

Hi all, 
My problem is the following: having a lot of nitrates, may I do water changes
(every 15 days) with RO pure till will not decrease?
My tank is 40g with yaest CO2 and Tetra drops for plants.
Thks & rgds
Ciao  Ezio                _ _
______________________,,,_o o_,,,_______________________
  FERRO Ezio                             Torino - Italia  
              Quando il gioco si f duro,
               il duro che inizia a giocare.