the nitrates are the end result of your bioload.. fish, shrimp, crabs, snails etc. and the feeding of them. the refuge or sump (mine is a combination made from a 30 gallon square rubbermaid container) is a place where some of this can be exported using macro-algae, mangroves and many other typer of marine plants. it is also another area to place a deep sand bed to help in denitrification. it will also in effect make your tank bigger with the increased water volume...
0 nitrates can be acheived with out doing this provided the tank is not overstocked and you have enough oxygen free areas ( in your sand bed and live rock) the bacteria that turn nitrate into nitrogen gas can only live and work in these areas...have you ever noticed small bubbles forming on the sand and when they get large enough float up to the top and pop. that among other things is the nitrate leaving your tank.
really as i see it to reduce the nitrates is more water changes with nitrate free water (test your supply)or reduce bioload, create more anerobic areas, have nutrient export,or a combination of the above..
a sump or refuge is only a tool to help with this....
hope i helped...balance is what must be achieved between what goes in and what comes out.
