The simplest way is through water changes. Regularly changing a significantly large proportion of the water in a marine aquarium can reduce the concentration of nitrate through dilution, keeping nitrate at acceptable levels. The required amount of water to be changed, and the frequency of the changes, will depend on the how much nitrate is being produced, and how much needs to be removed. A lightly stocked, lightly fed tank will naturally have lower nitrate levels than a highly stocked and/or overfed tank.

Another way to remove nitrate from an aquarium is to grow algae (commonly macro algae) which use nitrate (and phosphate, though that is unrelated to the Nitrogen Cycle) to grow. These algae can then be "harvested", removing the nitrate they have used to grow from the aquarium. This is normally done in a macro algae refugium or an algae turf scrubber (ATS).

Another way nitrate can be removed is by converting it to nitrogen (a harmless, inert gas) which simply exits the aquarium as part of normal gas exchange at the water's surface. This can be done with the use of a deep sand bed, or DSB, which is a bed of sand at least 10cm (4") deep, in which the water is poorly oxygenated enough due to low water flow that anaerobic sections (areas without any oxygen present) can form. There are bacteria which live in anaerobic conditions which can convert nitrate into nitrogen. A variation on this method involves the use of a plenum, or small space of still water, underneath the sand bed - again, this water becomes so poorly oxygenated due to bad flow that it becomes anaerobic, allowing the bacteria which convert nitrate to nitrogen to grow.

Sulphur/Nitrate Filter

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