Although less toxic than ammonia and nitrite; nitrate (NO3) as a nitrogen compound also causes stress at all levels making a fish’s organs work harder to adjust to it’s new environment, especially at levels higher than 100 ppm). The increasing stress results in the loss of ability to fight diseases, the ability to heal itself, and the ability to reproduce.
It is essential for you, the aquarium hobbyist, to maintain a proper environment for your aquatic companions. High nitrate levels are a sign of poorly maintained aquariums and will cause problems in the long run (I recommend levels under 40 ppm for FW, 20 ppm for Saltwater).

Nitrates are potentially dangerous due to the effects on the water chemistry and on a healthy environment for your fish while nitrates are accumulating. The higher the nitrate levels the higher and severe the consequences due to the stress on your fish and the favorable conditions for a serious algae outbreak.
Nitrate levels around 5 ppm or less are found in nature which provides an almost nitrate free environment (although a Nitrate level of around 15 ppm is usually best for planted freshwater aquariums). The higher the nitrate concentration the more stress for the fish. Extremely severe stress is reached at levels exceeding 100 ppm. Many plants will fail before reaching this level, this due to an accumulation of life forms such as detritus worms feeding on decomposing waste, and the consequently higher biomass (organisms living in the aquarium) leads to an increasing demand of oxygen.

I will also add that although nitrates are not dangerous in the short term unlike ammonia or nitrites; in established tanks I usually test this parameter more often as this is a good indicator of how well I am doing in my tank cleanings (if enough water is being changed and often enough). Tests of KH & GH are also useful in indicating an established tanks health.

One more point about nitrate; I have tested the water on under sized aquariums /bowls containing otherwise healthy goldfish. The nitrates would often exceed 200 ppm! These goldfish (although they appeared healthy), rarely lived more than 3-5 years as compared to the 12 + years of the goldfish I have kept for clients in pond and larger aquariums.
University studies in Cattle show nitrate levels in water over 221 ppm to be harmful or even fatal, so I sure would NOT want my fish kept at nitrate levels approaching this number.

*WHAT ARE NITRATES?

Nitrates (NO3) are compounds composed of a nitrogen and three oxygen atoms and are often the final stage in the nitrogen cycle of fresh and saltwater aquariums if there are not nitrate removing plants, algae, or anaerobic bacteria present. Nitrates are the conjugate base (chemical substance that releases a proton in the backward chemical reaction) of nitric acid (HNO3), consisting of one central nitrogen atom surrounded by three identical oxygen atoms. The presence or production of large amounts nitrates can result in the presence of Nitric acid according to the Brønsted-Lowry theory of acids and bases which will in turn affect an aquariums pH and KH (which can result in dangerous pH swings).


*HIGH NITRATE DANGERS:

High nitrate levels which many sources based in human studies place as low as 30- 45 ppm of nitrate as harmful. The EPA recommend levels of 10 ppm in drinking water, although often this can higher especially in well water.
High nitrate levels can cause respiration problems in fish, lower or eliminate the ability to breed, resist disease, and lower activity of aquarium inhabitants. In human studies high nitrate levels have been shown to dangerously lower blood pressure by causing the muscles that control the size of blood vessels to relax, this can be dangerous to fish too causing circulatory problems which can again result in poor disease resistance.
High Nitrate levels in aquariums will also result in high algae growths and in marine aquariums is toxic at even low levels to Cephalopods such as Octopus and to corals.

Another danger is in the bloodstream, nitrates can be converted biologically to nitrites, leading to "Brown Blood Disease".

For more on the dangers of Nitrates:
http://www.ext.vt.edu/pubs/forage/404-163/404-163.html


*REMOVAL / PREVENTION:

Here are a few basics for removal/ prevention of nitrates (I will add to this list over time too)
• Water changes; for high nitrate levels changing as much as 60% then filling the aquarium only 80% (this cuts the nitrates in half), followed by a 50% change again which will then have an over all reduction of 75%.

• In Freshwater; plants will keep lower nitrate levels.

• Vacuuming of eventual nitrate producing mulm is essential in both freshwater and saltwater as long as this performed correctly so as to not disturb anaerobic bacteria in saltwater or plant roots in freshwater, please see this article: “Aquarium Cleaning”

• Lower you bio load, reduce feeding and use foods that are more easily digested (made with amino acids that will be used by the fish/aquatic organism and not be expelled) such as Spirulina Algae based foods

• In marine aquariums the use of live rock, mud filters/Refugiums and/or protein skimmers can help reduce nitrates. Mud filters and Refugiums remove nitrates while Protein Skimmers remove nitrogenous wastes (proteins) before they enter the nitrogen cycle. I personally feel protein skimmers can be over rated due to the often erratic upkeep they require. I prefer the more natural use of live rock and mud filters/Refugiums as well as Mangrove Plants and Caulerpa algae in my marine aquariums (this is not to say I do not use protein skimmers, as I do. I have also kept many a marine aquarium with very low nitrate levels without these faddish devices). Good deep sand (#00) beds of over 2” also are helpful as these allow anaerobic bacteria to form and remove nitrates.

• Use a recirculating micron cleaning filter to super clean your aquarium without over cleaning (meaning excessive water changes over 50%). The Aquarium Cleaning Machine is an awesome device for this purpose.
The key is this machine (& similar, but this is the best by I have used) VERY efficiently removes fine organic particulates BEFORE they would otherwise go through the nitrogen cycle. Rinsing in of cartridges in CHLORINATED water after use prevents the formation of nitrifying bacteria that would break down organics resulting in higher nitrates. Despite some naysayers in some forums, this machine is very efficient for this purpose when use correctly and it does not simply re-circulate DOC (dissolved organic compounds) through the water, it removes much of the small organic particulates before they enter the nitrogen cycle thus becoming nitrates. The theory is similar to how a protein skimmer works in marine aquariums, however this machine and similar can be used in salt OR freshwater!


• Rinse filters often with de-chlorinated water to prevent mulm build up. This is especially important in “nitrate factory” filters such as Wet-Dry and canister filters. The rinsing of sponge filters is also important. In heavily planted freshwater aquariums this is often not necessary as the plants will often consume most extra nitrates and over cleaning can actually reduce nitrates too low for heavily planted tanks with CO2. However in marine aquariums this IS essential.
This is especially important with canister filters and Wet/Dry filters which have the reputation within the saltwater side of the aquarium keeping hobby as “nitrate factories” due to the fact that these filters both trap a lot of organic debris and also are very efficient aerobic nitrifying filters yet do little for de-nitrification. Rinsing bio media is helpful for these filters as well as increasing frequency of this procedure, this goes for freshwater as well. I have add success in preventing this problem with the use of live rock crumbles in place of much of the standard bio filter media in saltwater aquariums so as to allow for de-nitrification deep inside these crumbles. Volcanic rock is a substitute that works well too for this and can be used in FW as well.

• Products such as Algone for freshwater or saltwater can aid in nitrate prevention/removal.

• There is also nitrate “sponge” or resins available, although these can be costly. Zeolite can be used in freshwater aquariums to remove ammonia before it ever goes thru the nitrogen cycle, eventually becoming nitrates.
Another popular product is SeaChem’s Purigen which can be used in both freshwater and saltwater. Purigen can also be regenerated with bleach, however do not use in combination with Stress Coat (for more about this please read this article: Aquarium Answers: Aquarium Water Conditioners ). Purigen can also raise your Redox, so I would not recommend continuous use for this reason.


• The use of fine poly filter pads inside easy to access filters such as aquarium power filters (HOB) or wet/dry filters. I would place these in easy to reach spots and NOT use them to replace the regular filters or sponges, rather place a piece in front of the existing filter. This pad needs to be rinsed every 1-3 days. How this works is this filter material will trap fine organic particles BEFORE going through the nitrogen cycle which would otherwise result in nitrates. The key is regular rinsing in CHLORINATED water so as to prevent the formation of nitrifying bacteria that would break down organics resulting in higher nitrates. This is similar to how the micron cartridges work (except on a much smaller and less efficient scale) in cleaning filters such as the Aquarium Cleaning Machines. I recommend generic versions rather than the more pricy name brands which can be cut to fit, here is one example of these generic pads: Aquarium Filter Media


• The use of a Protein Skimmer in marine aquariums is an effective way (depending on the skimmer) of removing ‘protein’ based organics BEFORE they can enter the nitrogen cycle and thus become nitrates. These devices do NOT work in freshwater as they work via foam refraction which is a process that will not work in freshwater.
Please see this article for more information about Protein Skimmers: Marine Aquarium Protein Skimmers



• Check your clean water source as I have tested Tap Water in excess of 20 ppm and some wells may also be high. The use of “clean water” that already has moderate amounts of nitrates lessens the effectiveness of water changes. For SW the solution is simple, I recommend the use of RO, DI or similar water for mixing all new saltwater and for topping off for evaporation. For FW, this is a bit more difficult as RO/DI water is missing VERY essential elements for fish osmoregulation so its use is not generally acceptable (the use of marine salt mix replenishes ALL necessary elements back to the water). At best the FW aquarist can uses partial RO, DI or similar and then reconstitute the water with minerals with products such as Wonder Shells, Alkaline Buffers, and Cichlid Salts.

• Carbon does not remove nitrates; Although the use of carbon supporters and detractors, one aspect of activated carbon that is clear is that it cannot remove nitrates. Activated carbon filtration does not remove microbes, sodium, nitrates, fluoride, and hardness.
Reference: http://www.ag.ndsu.edu/pubs/h2oqual/watsys/ae1029w.htm

Please reference this other Aquarium answers artilce for more about Activated Carbon and other filter media types as well:
Aquarium Filter Media; types, capacities and more


For a map of Nitrates in World Rivers, please click below
Nitrate Levels in Major World Rivers

For more freshwater and marine basics, please visit this Aquarium Information Resource