Hi,
Nitrate is a simple yet complex problem to any new or experienced aqurist, it is part and patial of reef keeping. If your tank is new, just be patient, try to balance the bio-load, that is the input and out-put, what ever food goes in or waste produced in the tank, the filtration system should be able to remove them. If your tank is more than six months, than you will have to check the filtration system whether your syetem is adequately designed.
In layman term, the reef tank should have two main areas of cultivating difference baterials for different function, aerobic and anaerobic, aearobic area is rich in Oxgyen, such as bio-balls, cotton wool, over-aged activated carbon, coral chips filtration etc. this is known as nitrat factory, cos theses areas break down amonia to nitrite and to nitrate.
While the anaerobic area is very low in Oxygen, these areas produce baterial that consume NO3, areas such as LR; lower part of DSB, denitrifier etc., we need to create enough anaerobic area to eliminate NO3 in order to keep the NO3 level low.
A lot of aquarists create more aerobic area but insufficient anaerobic area in their reef tanks, that casue the rise of NO3.
Dr Jaubert's plenum system creates massive anaerobic area in the plenum area which has proven to be a great success in reef keeping.
Using good skimmer to remove potein and nutrient in the tank will also help to keep the NO3 level low.
Hope can help u to solve the promble.
Cheers!
Chipson1