Nitrate In The Aquarium

As discussed in our page on the Nitrogen Cycle, nitrifying bacteria break down toxic nitrite (NO2) to nitrate.  Nitrate in the aquarium is not toxic like ammonia and nitrite, except at high levels above 40 ppm for invertebrates and above 100 ppm for fish.

Nitrate And Small Polyped Stony corals (SPS)

SPS corals for example will start to turn brown when nitrate in the aquarium reaches 20 ppm.  This browning is caused by their zooxanthallae in the corals tissue absorbing nitrate.  Zooxanthallae are the symbiotic algae in the tissue of corals and clams that produce sugars as a by-product of photosynthesis.  These sugars are the food source for all photosynthetic corals.

At anything above 30 ppm of nitrate, SPS corals will expel their zooxanthallae, resulting in the coral bleaching.

Conventional wisdom of the past told us to maintain levels of nitrate at zero for SPS corals.  Modern SPS keepers however find that a low nitrate level of around 5 ppm makes the colors of SPS corals more vivid!

Nitrate And Large Polyped Stony Corals (LPS)

As a general rule, nitrate in reef tanks with LPS corals should be kept  below 25 ppm, although many aquarists happily maintain their LPS tanks at between 25 to 40 ppm .

There is evidence that certain corals like the Elegance coral (Catalaphyllia jardinei) actually prefer high nutrient environments with nitrate at around 25ppm.  These corals are found in low current, lagoon sandbeds where there is much silt and mud.

Elegance corals deteriorate and die when there is insufficient nitrate in the aquarium.  The effect of elevated levels of nitrate on corals depends very much on the species of coral.

Nitrate Levels For Fish-Only-With-Live-Rock (FOWLR) Tanks

Fish have a far higher tolerance for nitrate than corals — 50 ppm of nitrate in fish only systems are not uncommon.  Although some herbivorous fish like tangs will stop eating when nitrate in the aquarium is too high.

But while fish will tolerate high nitrate levels, nuisance algae also profilerates when there is too much excess nitrate.  Which is why we see so many FOWLR tanks with a moss-like carpeting of green nuisance algae all over the live rock!

For the health of our fish, as well as to keep nuisance algae at bay, we should strive for nitrate levels of around 25ppm in FOWLR aquariums.

Nitrate And Clams

No, it’s not a new seafood recipe..

Clams are known to consume ammonia and nitrate which is one of the reasons why they are so favored by SPS reefkeepers.

And like SPS corals, clams also require high intensity lighting for their zooxanthallae, making them well-suited to an SPS environment.

How To Reduce Nitrate In The Aquarium

High nitrate in the aquarium is usually the result of overstocking, overfeeding and an inefficient biofilter, exacerbated by a lack of regular water changes.

Some of the best ways to reduce nitrate in aquariums are:

  • Avoid overfeeding — as discussed on our page on Feeding Marine Fish, high nitrate levels occur when excess food is not consumed and left to decay in the aquarium
  • Avoid overstocking — too many fish lead to a large amounts of excreted waste.  In FOWLR tanks where fish are the main focal point, it is difficult to resist the temptation to overstock.  Adding some hardy leather corals to a FOWLR tank helps reduce the temptation to add too many fish to make the tank look more interesting!
  • Keep up with regular water changes — a 10 to 15% water change every week, or at the very least, every month, is one of the best ways to keep nitrates low
  • Ensure efficient aquarium filtration — having enough live rock for denitrification, and running a protein skimmer helps keep nitrate in the aquarium low

More on Aquarium Water Parameters:

Ideal Water Parameters For A Reef Aquarium

pH And Alkalinity

Ammonia And Nitrite

Calcium In The Reef Tank

Phosphate and Phosphate Adsorbing Media

New Tank Syndrome

Water Parameters | Danger Levels

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