How to Lower Nitrate in the Aquarium

Freshwater Aquarium
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The significance of nitrate in the aquarium is arguably less understood by fish keepers than the effect of ammonia and nitrite. Although nitrate is not as directly lethal as ammonia or nitrite, over time, high levels of nitrate negatively impact fish and the aquarium environment in general. 

Where Does Nitrate Come From?

Nitrate is a by-product of nitrite oxidation during the latter stages of the nitrogen cycle and is present to some degree in all aquariums. Detritus, decaying plant material, dirty filters, over-feeding, and overstocking the aquarium all contribute to increased levels of nitrate.

Additionally, tap water used to fill the aquarium may contain nitrate in it. In the United States, drinking water may have nitrates as high as 40 parts per million (ppm). Before adding water to your aquarium, test it for nitrate to discover if the levels are unusually high in your water source. If your baseline nitrate is above 10 ppm, consider other water sources that are nitrate-free.

Desired Level

In nature, nitrate in water remains very low, generally well below 5 ppm. In freshwater aquariums, nitrates should be kept below 50 ppm at all times, and preferably below 25 ppm. If you are breeding fish, or are battling algae growth, keep nitrate even lower, below 10 ppm.

Effect on Fish

Fish will feel the impact of nitrate by the time levels reach 100 ppm, particularly if these levels persist. The resulting stress leaves fish more susceptible to disease and inhibits their ability to reproduce.

High nitrate levels are especially harmful to fry and young fish and will negatively affect their growth. Furthermore, the same conditions that cause elevated nitrate often cause decreased oxygen levels, which further stress the fish.

Nitrate and Algae

Elevated nitrate is a significant contributor to undesirable algae growth, and nitrate levels as low as 10 ppm will promote algae growth. The algal blooms that are common in newly set-up tanks are usually due to elevated nitrate levels.

Although plants utilize nitrate, if nitrate levels rise faster than the plants can use them, then even the plants can become overgrown with algae, ultimately leading to their asphyxiation and demise.

How to Reduce Nitrate

Unlike the aerobic bacteria that convert ammonia to nitrite and then nitrite to nitrate, the bacteria that remove nitrate avoid oxygen-rich environments. Therefore, well-oxygenated conventional filters, unfortunately, will not harbor the kinds of bacteria that remove nitrate.

However, there are some steps you can take to keep nitrate low.

  • Keep the aquarium clean: Waste ultimately produces nitrate; cleaner tanks produce less nitrate that must be removed by water changes.
  • Feeding amounts: Overfeeding is a significant contributor to excess nitrate and other undesirable wastes, such as phosphate.
  • Water changes: Performing regular water changes with water that has little or no nitrate will lower the overall nitrate level in the aquarium. If your local tap or well water is high in nitrate, using deionized water (DI) or reverse osmosis water (RO) can help keep nitrate levels low when doing a water change. However, since these are devoid of minerals, the hardness, alkalinity and pH of the water can become too low and mineral supplements may need to be added. Mix your nitrate-containing tap water with DI or RO water to make a blend with the correct water parameters.
  • Keeping live plants: Live plants utilize nitrate and will help keep the levels lower.

Although special filters, called denitrators, exist that will remove nitrate, such devices are usually quite expensive compared to other filtration units. Instead of purchasing a pricey denitrator or special filter, you can purchase from your fish store a nitrate-lowering media to put into the filter you have. These will pull the nitrate out of solution, and need to be replaced periodically.