Berrykin..my tank something like yours. I have never tested the nitrates till now...so I'm not overly concerned. Anyway...I found out from past experiments of mine last time that changing 50% of the water does not equivilate to removing 50% of nitrates. It does'nt work that way. It might only end up removing 10% or less of your nitrates.
You might want to try AZ-NO3? Or BioHome..which is what I am using..have not tested how efficient it is at nitrate removal though.