How to Lower Nitrates in the Aquarium

Aquarium nitrates
Shannon Ramos / EyeEm/ Getty Images

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

Where Do Nitrates Come From?

Nitrates are a by-product of nitrite conjugation during the latter stages of the nitrogen cycle and are present to some degree in all aquariums. Detritus, decaying plant material, dirty filters, over-feeding, and overstocking the tank all contribute to increased levels of nitrates.

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

Desired Level

In nature, nitrates remain 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 nitrates even lower, below 10 ppm.

Effect on Fish

Fish will feel the impact of nitrates 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 nitrates often cause decreased oxygen levels, which further stress the fish.

Nitrates and Algae

Elevated nitrates are 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 setup tanks are usually due to elevated nitrate levels.

Although plants utilize nitrates, if nitrates 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 Nitrates

Unlike the bacteria that consume and convert ammonia and nitrites, the bacteria that remove nitrates avoid oxygen-rich environments. Therefore, well-oxygenated conventional filters, unfortunately, will not harbor the kinds of bacteria that eat nitrates.

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

  • Keep the tank clean: Waste ultimately produces nitrates; cleaner tanks produce fewer of the nitrates that must be removed.
  • Feeding Amounts: Overfeeding is a significant contributor to excess nitrates and other undesirable wastes, such as phosphates.
  • Water changes: Performing regular water changes with water that has little or no nitrates will lower the overall nitrate level in the tank. Pure deionized water (DI) or reverse osmosis water (RO) are excellent choices for keeping nitrate levels low through water change procedures.
  • Keeping live plants: Live plants utilize nitrates and will help keep nitrates in check.

Although special filters, called denitrators, exist that will remove nitrates, such devices are usually quite expensive compared to other filtration units. Instead of purchasing a pricey denitrator or special filter, you can use nitrate-lowering media in the filter you have. Although these mediums will not lower nitrates dramatically, when used together with other media, the net result can be markedly beneficial.