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[edit] What Is Nitrate?

Nitrite (NO2-) is transformed by a genus of bacteria called Nitrobacter to nitrate (NO3). In an oxygen environment, nitrate cannot be broken down any further. Anaerobic bacteria (bacteria that live in the absence of oxygen) can break down nitrate to nitrogen gas. This process takes place in deep sand beds or inside of porous rock.

Nitrate is relatively harmless to marine fishes, but persistent levels may be an indication of other difficulties. Indeed, I have found that aquariums with high nitrate levels usually have deeper problems.

[edit] Why Test Nitrate?

Although nitrate may be more-or-less harmless in concentrations below 40 ppm, it can be an indication of more generally declining water conditions. The aquarist may need to perform a thorough evaluation of any aquarium which maintains consistently high nitrate levels (10 ppm or greater). Nitrate should be tested at least once per month.

[edit] How To Test Nitrate

Various nitrate test kits are available to the marine aquarist. Some make testing as simple as dropping a couple of tablets into a sample of water, others are more complex multiple-part tests. Whichever you choose, simply follow the manufacturer’s instructions and follow good laboratory procedure. Note: Some kits test nitrate and some test for nitrate-nitrogen. If your test is for nitrate-nitrogen. Multiply the result by 4.4 to get nitrate.

[edit] Proper Nitrate Level

Nitrate levels should be maintained at or near zero. Even though nitrate appears to be harmless in low concentrations, a nitrate level below 10 ppm for fish-only tanks and below 2 ppm for reef aquariums is recommended.

[edit] Record Nitrate in Your Log

Once you have obtained the result of the nitrate test, record it in your log and on your graph. Note that consistent increases in nitrate levels may indicate generally degrading water conditions..

[edit] How To Lower Nitrate

Clean all filters to remove clogs and decaying matter; do a 20% water change to reduce the nitrate concentration. Regular general maintenance and sticking to the basic husbandry methods put forth in this book will go a long way toward the elimination of nitrate as a problem source. Other methods to reduce nitrate levels:

  • Avoid overfeeding.
  • Avoid overstocking. (Large fishes can overwhelm a home aquarium with their wastes.)
  • Increase water circulation.
  • Increase skimming. Add a more efficient skimmer to the system.
  • Increase frequency/volume of water changes.
  • Add a lighted refugium or sump with macroalgae to extract nitrate and other dissolved organics from the water.
  • Add porous (completely cured) live rock or a deep sand bed (4 or more inches) to cause some nitrate to be converted into nitrogen gas. The resulting nitrogen gas will escape into the atmosphere.

Carl DelFavero
Reference: Aquarium Keeping and Rescue