You first need to tell us aboout your tank.
How many gallons/ litres is the tank?
How long has it been up and running with fish?
What are the current water parameters? Ammonia, nitrite, nitrate, pH, salinity, temp, calcium and alkalinity. Please post numbers of results.
What else is in the tank (livestock and equipment)?
What is the nitrate problem you are having? What are your nitrate numbers?
How often do you do a partial water change and how much water do you change each time?
What type of food are you feeding (frozen [what?], flake [what?], freeze dried [again, what?])?
Are you adding any supplements?
If so, what, how much and how often?
For a FO
tank, PWCs are your best choice.