Keeping nitrates in check can be one of the most frustrating (and expensive) long term task a saltwater aquarist faces. Considering the price of a bag of sea salts, the cost in dollars (let alone the time) of repetitive water changes can add up to a tidy sum over time.
Are there any viable alternatives to constant water changes to reduce or eliminate nitrates? Thankfully, the answer is "yes". Not overfeeding and removing uneaten food and detritus from the tank is a good start. The use of Mangroves in either the tank or sump is also a reliable method.
Another reliable, but heretofore unheralded method in the saltwater aquarium hobby is the use of the combination of Hiatt's Right Now! (RN!) bacteria and a quantity of carbon (we used Hiatt's Tri-Based Pelletized Carbon (TBPC) with great success) in the tank's filtration system. (See the results in "New Biological Filtration For an LFS - 1,000 Gallon System" and "New Biological Filtration For an LFS - 3,500g Shark Tank"). There is a specific bacteria strain in the RN! which reacts with Nitrates and an element in the TBPC, converting the nitrates into Nitrogen gas, which is then vented into the atmosphere via the water's surface.
The TBPC is also an excellent biological filtration platform. The huge (Over 1150 m2/gram) surface area is far superior to anything else on the market. Hiatt recommends .1669 lb. of TBPC per gallon of system water to achieve proper biological filtration and nitrate reduction. However it has been our experience (over 15 years) with the products, that a far less quantity is required to achieve satisfactory results. We currently have several small (75g) tank projects running at Animal Jungle (in Kinston, NC) which use 1.40 lb of TBPC (in 2 4" X 12" bags per system) in the center of Aqua Clear canister filters, pushed by 1200 gph pumps. To date these filtration systems are achieving all of our system goals (0 ammonia, 0 nitrites, 0 nitrates).
You can also use the Vodka Method for reducing nitrates. While this method does require adding a dose of Ethanol (Vodka, if you wish) to your tank every week or so, it appears that this method is also effective at reducing nitrates aerobically.
Most reef aquarists utilize additives (calcium, iodine, pH buffers, trace elements, etc.) on a regular basis to enhance their saltwater properties for maximum coral and other critter health and growth. The use of RN! and TBPC in a filtration system doesn't change this. The only thing it seems to change is the need for constant water changes to reduce or eliminate nitrates in the system and the requirement for a bulky biological filtration platform to achieve desirable results.
Hiatt products can be found at a number of LFS, as well as online at Memory Doctor.