There are ways of removing Nitrates in a seawater aquarium.
I have a completely natural system, no skimmer, no canister filters, no UV.. just Natural seawater, liverock and Mineral mud (in the sump with caulerpa).
Caulerpa (and any algea for this matter) is excellant at removing Nitrates. Caulerap is usually used as it grows very aggressively and thus utilises more nitrate than other slower growing algeas.
Live rock is another excellant way of removing Nitrate.
To move the Ammonia cycle along its natural path you have to have an environment that is Oxygen rich for the bacteria to use when converting Ammonia to nitrite and nitrite to nitrate.
Canister filters are considered by many to be poor filters and nitrate factories as they build up alot of bacteria that turn out alot of nitrates. The problem here is that they use alot of the oxygen in the water and thus deprive the fish of their much needed oxygen (as opposed to trickle filters that uses oxygen from the atmoshpere.).
Many still use cannister filters though and to be honest i have never seen much problem with oxygen levels. As liong as the tank has a large turnover of water then there should be good oxygen exchange.
Anyway.. I digress...
TO be able to finish off the Ammonia cycle and thus convert Nitrate to Nitrogen gas to be released into the atmoshpere (and thus rendering the total safe end of the ammmonia cycle). the bacteria that do this work live in an oxygen poor envirnment. Thus this means that almost any filter will fail to accomplish this task as they are designed to be oxygen rich. Live rock on the otherhand is porous, on its outter surface it has great colonies of bacteria that utilises oxygen and thus start the natural cycle. However, as this water passes deeper into the rock, its oxygen is reduced significantly until its alomst deprived of oygen altogether in the center of the rock.. This is where the most precious bacteria that can render an end to the cycle "hang out" in great numbers. Good quality live rock will be a great filter for finishing the cycle and turning Nitrate to Nitrogen gas.
I hope this helps clear up the way that Nitrates are created and handled in a Seawaer aquarium.
Cheers
Navarre