I had written this for somewhere else and its applicable here. Maybe you'll find this useful
Reduction of nitrates in your fishtank
Determine the factors that contribute to your nitrate problem such as: high fish bioload, overfeeding, reduced frequency/volume of water changes, and water source. Changing any of these factors can help. As a quick fix, increase you frequency and volume of water changes. However long term, look towards some form of increased nutrient export such as including a remote sand bed, refugium, or combination.
Below are a list of common factors that contribute to increasing nitrates in your fish tank, and my recommendation on how to reduce it.
Overfeeding- the largest input source of nitrogen in your tank is food, so feeding less will be supplying less input nitrogen- do this within reason.
Reduce bioload- more fish equal more fish waste. Reducing a few fish will go along way to increasing water quality. This concept is critical in small tanks with little water volume.
Increase water exchanges- more frequent and consistent water changes will remove and dilute existing nitrates.
Export nutrients by skimming-or increase your amount of skimming. While excessive nitrates are not directly removed by skimming, adjusting your skimmer to collect slightly wet or wet foam will remove many particulate and DOC waste before its converted, additionally, increased skimming will increase oxygenation
Export by growing/harvesting macroalgaes- plants prefer to use ammonium compounds for fertilizer, and can be purposefully planted in the sump or refugium. The more you grow and harvest, the more nitrogens will be exported
Add a deep sand bed or a remote sand bucket- Sandbed are becoming quite common place in the home aquarium and using a deeper bed (>4”) facilitate the creation of anaerobic zones in which anaerobic bacteria can reduce nitrate into nitrogen gas and O2. These deep sand beds can be placed in your sump or refugium or used a remote deep sand bucket. This concept pioneered by Anthony Calfo (a noted author in the marine aquarium hobby) uses a bucket 2/3 filled with sugar-sized sand and plumbed inside or outside your sump. A small powerhead (50-200gal/hr) pumps water into the bucket, and nitrates are processed at the depths of this bucket. This bucket concept is extremely useful to transfer from tank-to-tank once established.
Remove existing wet/dryes or filter media- the wet/drys do such a great job at converting ammonia to nitrite ad nitrate, but do nothing with the accumulating nitrates (which requires anerobic bacteria to process). Many hobbyist have had success with slowly removing the filter media in the wet/drys and allowing more of the nitrogen cycle processing to take place on the live rock and in the sand beds
Denitrators- A number of commercial systems are available, and these are becoming popular with hobbyists. The two most common denitrator types use carbon or sulfur, in which bacteria use methanol or elemental sulfur and produce nitrogen
Nitrate absorbing compounds- various aluminum oxides and zeolite products (kent nitrate sponge)
Polymers and carbons that bind organics
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frank
