The Adverse Effect of Nitrate on the Aquarium

By Tony Griffitts

The effect of nitrate on the aquarium and pond system has not been very well understood in the hobby over the years.  In fact, it was often considered harmless.  We now know it is not harmless and is the most likely cause of Hole in the Head disease in cichlids, and Head and Lateral Line Erosion in Marine fish.  If it is allowed to remain unchecked in the system, death can eventually occur.

First a review of how nitrate accumulates in the aquarium.  In general, fish excrete ammonia through their gills and waste.  Ammonia is then oxidized by Nitrosospira and Nitrosomonas bacteria into nitrite, and then Nitrospira bacteria oxidize nitrite into nitrate.  In most hobbyist aquariums nitrate accumulates over time, depending on the bio-load and any nitrate reducing systems that have been implemented.

Very little if any research on the effects of nitrate in the aquarium or pond system have been conducted.  Most of what we know about the effects of nitrate are through long term observations by advanced hobbyists.  There is some research that has been conducted on  nitrate with cattle and humans, and because of this research the Environmental Protection Agency has set the Maximum Contaminant Level (MCL) for drinking water at 10 ppm.  Nitrate when consumed by human infants and cattle gets converted into nitrite in the digestive tract by anaerobic bacteria.  Nitrite in the digestive tract is adsorbed by the red blood cells, displacing oxygen, and creating Methemoglobin.  Methemoglobin turns the blood brown, because it is no longer carrying oxygen.  This condition is known as methemoglobinemia.  If nitrate ingestion is not discontinued suffocation (death) will occur.

Blue Tang (Paracanthurus hepatus) with Marine Fish Head and Lateral Line Erosion due to exposure to water that had a nitrate level above 400 ppm.

In my experience working with aquariums and troubleshooting aquarium problems for other hobbyists, excessive nitrate in the aquarium is the cause of Hole in the Head (HITH) disease in cichlids, and Head and Lateral Line Erosion (HLLE) in Marine fish.  In every case I have worked with, high nitrate, often over 400 ppm was present.  Hole in the Head, and Head and Lateral Line Erosion has been suggested to be caused by a lack of vitamins, poor nutrition, stray voltage, use of activated carbon, and internal parasites (Hexamita/Spironucleus).  In cases where fish had proper nutrition, and the aquarium had no stray voltage, and did not use activated carbon in the filtration system, fish still developed HLLE in high nitrate water.  In cases of cichlids, I have donated lots of apparently healthy and unhealthy Discus for fish disease research, and in most cases both apparently healthy and unhealthy fish had Hexamita/Spironucleus in the digestive tract, but never exhibited HLLE.  In my experience it is possible to reverse Marine Fish HLLE by moving the fish to a system that has less than 40 ppm nitrate.   I have been able to reverse the condition in clownfish (A. ocellaris), and several species of tangs.  The process can take months depending on the species, but it is also possible to completely reverse condition.

How does nitrate cause the HLLE and HITH?  Based on what we know about the nitrate research on other animals,  the Griffitts theory on nitrate toxicity in fish works like this:

• When fish feed in systems with high nitrate water they take in small amounts of water containing nitrate.
• Anaerobic bacteria in the digestive tract break down the small amount of nitrate into nitrite.
• Nitrite is adsorbed by the the red blood cells of the fish, creating a small amount of Methemoglobin.
• The small amount of Methemoglobin is not enough to kill the fish, but causes a chronic low oxygen level in the circulatory system.
• The fish's tissue uses the oxygen, but by the time the blood gets to the outer extremities of the head and the lateral line, only Methemoglobin remains in the blood.
• Tissue around the head and/or lateral line will start to die do to the lack of oxygen in the blood.

Nitrate also has an adverse affect on reef invertebrates, especially clownfish host anemones .  Many fish breeders also have noticed that nitrate can be a growth inhibitor and also stop reproduction.

When maintaining an aquarium or pond it is recommended that you keep the nitrate level below 100 ppm for a fish only system, below 40 ppm for most reef tanks, and below 20 ppm for systems that house host anemones.

There are several methods of controlling nitrate:

• Denitrator.
• Deep sand bed (2 more inches (5 cm).
• Macro algae and aquatic plants.
• Bright light.
• Water changes.

A denitrator uses anaerobic bacteria to reduce nitrate to nitrite and then to nitrogen gas.  It works by flowing water slowly through a filter with a lot of surface area for bacteria to colonize.  Aerobic bacteria near the front of the filter use up the oxygen in the water, creating an environment for anaerobic bacteria to do their work reducing nitrate.

Deep sand bed in marine systems help reduce nitrate by providing a place for anaerobic bacteria to colonize.   Spaghetti worms (tube worms) and other invertebrates that colonize the sand bed help exchange water slowly through the sand bed allowing anaerobic bacteria to reduce nitrate to nitrogen gas.  Generally you need to have at least a 2 inch (5 cm) fine sand bed to see some de-nitrification.

Macro algae and aquatic plants take up nitrate as a nutrient.  Many reef aquarist use macro algae in a refugium to take up excess nitrate.  Reef systems that have macro algae often have an undetectable nitrate reading.  Heavily planted aquariums with bright light can also help control nitrate.  Ponds with lilies, cattails, and other aquatic plants that get a lot of sunlight often have an undetectable nitrate level, even when heavily stocked with large koi.

When nitrate accumulates in the system over time, water changes need to be done to reduce the nitrate level.  How fast nitrate accumulates in the system will determine how much and how often water changes need to be done.  In some cases, near 100% water changes may need to be done monthly or more often to keep nitrate within safe levels.  As long as pH, salinity, and temperature of the new water are close to what is in the system, large water changes are perfectly safe.

There are many products on the market that claim to reduce nitrate.  Some are liquid chemicals, and a mineral rock like zeolite.  None of the products I have ever tested worked as claimed.  If nitrate reduction was accomplished with these products, it was so small it could not be detected. 

Keeping nitrate in check is very important in having long term success with an aquarium or pond system.  Test your system periodically for nitrate, and establish a water change schedule to keep nitrate below acceptable levels.  Long term fish health depends on you maintaining a low nitrate level.