Nitrates in a salt water aquarium are a natural product of the nitrogen cycle. When this cycle is functioning properly, nitrites and ammonia are converted into small amounts of nitrate. Nitrate levels are usually low and are often filtered out. However, these levels can sometimes build up. High nitrate levels can be toxic, and can poison your fish. Nitrates can also slow growth in coral and fish, and can cause algae to grow in the aquarium.
Filter and purify all water you will add to the aquarium. Tap water can have high levels of nitrates in it, and can thus add nitrates to your aquarium.
Perform partial water changes frequently. Water changes keep the nitrogen cycle functioning while removing excess nitrates. This process lowers nitrate levels over a period of time.
Clean away as much organic waste material as you can. Organic waste in a aquarium produces nutrients, such as nitrates. Although aquarium plants need these nutrients to feed on, if you have excess nitrates, chances are your plants are not using all the nutrients and nitrates being produced by the extra waste.
Stop overfeeding your fish. Organic waste is often caused by fish food that has not been eaten and has begun to decay in the aquarium.
Add live rock to the aquarium. Live rock is full of microscopic organisms and bacteria that help the nitrogen cycle function properly. These organisms and bacteria essentially eat nitrates, and can therefore help to reduce nitrate levels. Add only a few pieces of live rock at a time so as not to unbalance the nitrogen cycle.
Add live sand to the aquarium. Live sand works in a very similar way to live rock.
Remove filters designed to aid in the nitrogen cycle. These filters convert nitrites and ammonia into nitrates. Removing them and allowing the live sand and rock to continue the nitrogen cycle alone can cause a more natural balance and reduce nitrates.
Introduce a denitrator unit. These units act as a natural nitrate removal system. They are full of nitrate eating bacteria, which, over a period of time, can help to reduce nitrate levels.
Add nitrate removing sponges or compounds to the salt water aquarium. These water additives are designed to soak up nitrates, along with ammonia and nitrites, which can cause high levels of nitrates.
Introduce nitrate fighting plants to the salt water aquarium. Many plant species act as effective nitrate sponges, soaking up nitrates and other excess nutrients and turning them into oxygen. These plants include Gracillarias, some types of algae plants, the Mermaid's Fan plant and some mangrove types.