Nitrates can build up in aquariums over time from uneaten fish food and fish waste. Nitrates can be toxic to aquarium animals, and they fuel nuisance algae growth. There are three basic ways to lower nitrates.

Water Changes
The great solution to all aquarium problems is a water change. It is important to remember your water change math when using water changes to reduce nitrates. Changing half the water will reduce nitrates by half, so if you have nitrates of 60 ppm and you want them to be below 5 ppm, you will need to do four 50% water changes back-to-back to get the levels to your target range. To avoid shocking your animals, we recommend changing no more than 50% of your water in 24 hours. Sometimes it may take a week of daily water changes to get your parameters in check. On a small tank this may be only a bucket of water and 10 minutes a day, but on a large tank doing daily large water changes can be impractical.

Further Reading. Aquarium Water Changes: Making It Simple

Algae Filtration
Algae use nitrate as a fertilizer to grow. Since we don’t want algae in the main display tank, we can instead grow algae in a refugium or algae turf scrubber. As the algae grows, it takes the nitrates out of the water to build its body. Once the algae has grown large, it can be harvested, and the nitrate is removed with the discarded algae. For saltwater aquariums you can add a grow light such as a Kessil H160 or Tunze Eco Chic to a sump chamber in order to grow macroalgae. If you don’t have a sump, you can add a Fiji Cube Hang-on Refugium to the side of the aquarium. In both freshwater and saltwater aquariums, you can add an algae turf scrubber like the IceCap Turf Scrubbers. They make both in-sump and hang-on varieties. These grow turf algae on a screen that can be periodically harvested.

Further Reading: Aquarium Media Reactor Overview

Bacterial Filtration
Another way of removing nitrate is bacterial filtration. There are naturally occurring bacteria that will break down nitrate and remove it from the aquarium water. But under normal conditions, they will not reproduce enough to effectively lower nitrate levels. You can increase the amount of nitrate-removing bacteria in our aquarium with organic carbon dosing. Organic carbon dosing provides the nitrate-reducing bacteria in aquariums with the nutrients needed to expand their population. As the bacterial population increases, the nitrate levels will drop. You can add organic carbon with liquid supplements such as Brightwell Aquatics Reef BioFuel, or you can use biopellets in a reactor. You can do this with Two Little Fishies NPX BioPlastics and a Lifegard Turbo Reactor. The organic carbon from the liquid or pellets feeds the bacteria, and then the bacteria removes the nitrate from the water. It normally takes about a month of organic carbon dosing to see nitrate reduction. You can control the amount of nitrate removal by the amount of liquid you add or the amount of pellets you keep in the reactor.

Further Reading: Aquarium Organic Carbon Dosing