Nitrates - How much is too much ?

[Toga]

Hi,

There is much debate within the hobby as to the maximum level of nitrate allowable in our tanks. Recommended numbers fluctuate anywhere from 100ppm to 400ppm, with 50ppm - 100ppm being generally accepted (not by me), and 20ppm being suggested for Nitrate sensitive fish. I personally, adhere to the less than 20ppm philosophy for any fish we keep.

Numerous articles & books have historically label nitrates as “harmless”. This has absolutely proven to be false. Nitrates are indeed toxic. The question is at what concentration does toxicity begin?

The problem with identifying a baseline level of recommended nitrate is it really depends on the species of fish. Some fish are designed by nature to withstand elevated nitrate levels and others will be impacted at very low levels, University of Florida on Gasmbusia in Florida Springs (3):

Quote:

Evidence suggests that sensitivity to nitrate is species-specific. Kincheloe et al. (1979) reported larval mortality of Chinook salmon, rainbow trout, and cutthroat trout at concentrations as low as 2.3-7.6 mg/L NO3-N. The 96-hr LC50 (median lethal concentration) for fathead minnow larvae is 1,341 mg/L NO3-N (Scott and Crunkilton 2000), and the lethal dose for adult and juvenile medaka is 100 mg/L NO3-N (Shimura et al. 2002).

A range of sublethal effects of nitrate has also been reported. For example, Greenlee et al. (2004) observed increased apoptosis and reduced cell number in cultured preimplantation mouse embryos exposed to 1 mg/L ammonium nitrate. In an accumulated nitrate test, in which nitrate built up over the course the experiment, Shimura et al. (2002) observed delayed hatching time and reduced fertilization and hatching rates of eggs produced by adult medaka exposed for 2 months to a maximum of 75 mg/L NO3-N. In that test, the offspring also exhibited reduced juvenile growth rates. At 50 mg/L NO3-N, Shimura et al. (2002) observed reduced spawning and fecundity (measured as egg number) among adult medaka exposed to nitrate as juveniles.

These studies identify that amongst three different species of fish, exposed to nitrates for 96 hours, one species of fish could only withstand 2.3-7.6ppm before death, another (the Fathead Minnow) 1,341ppm, and another 100ppm. These levels of nitrates can kill quickly (like ammonia), so we at least have confirmation that dispels the myth that nitrates are not toxic. "A range of sublethal effects of nitrate has also been reported".

So we know this, at least when it comes to short term exposure to very high levels of nitrate, there is a wide ranging concentration of what would be considered lethal and we know that even if not at “quick kill” levels, nitrates have a physiological impact.

Long Term vs. Short Term
I believe a lot of the confusion associated with suggesting maximum nitrate levels is that there is a vast difference between levels of nitrate that are determined to be lethal and levels of nitrate determined to be safe for long term exposure. As an example, the MDL for adult medeka fish is 96 hours at 100ppm while the maximum level considered safe for long-term exposure is less than 25ppm (7).

Physiological Effects of Nitrates
Some detailed studies on the affects of nitrates are listed below:

Research conducted for NASA designed to identify requirements for the long term raising of Medaka fish in space recommends keeping nitrates below 25ppm.
Studies performed on Gambusia in Florida springs discovered that decreased fertility rates were caused by nitrate in concentrations as low as 1.5ppm.
A recent study which reviewed all prior studies on the impacts of nitrates suggests that the most sensitive freshwater invertebrates and fish are affected by nitrate concentration as low as 2ppm, with the primary physiological impact being a decreased ability of the blood to carry oxygen (anemia).

So what physiological impacts do high levels of nitrate have on fish? One study conducted by the Virginia-Maryland Regional College of Veterinary Medicine on Hybrid Striped Bass detailed the physiological impacts of elevated nitrate.

This test identified that nitrates at 200ppm can kill relatively quickly. Within one week of being exposed to nitrates at this level the fish became blind and they began dieing seven weeks into the experiment. Autopsies revealed elevated nitrate concentrations resulted in the following physiological impacts:

Affects antibody production
Increased number of immature red blood cells
Lowered level of mature red blood cells (anemia)
Higher count of monocyte (a specific white blood cell)
Higher count of neutrophil (a specific white blood cell that is especially destructive to microorganisms)
Higher count of TLC - Thrombocyte-like cell (a blood cell of nonmammalian vertebrates that promotes blood clotting)
Higher levels of creatine (A nitrogenous organic acid found in muscle tissue that supplies energy for muscle contraction)
Higher calcium values in the blood
Lower Chloride values in the blood
Autopsy revealed damage to the spleen, liver, and kidneys

Other conclusions reached:
Nitrate damages the gills and kidneys affecting osmoregulatory ability (ability of the fish to regulate fluid levels and release toxins, something we do via urination, something they do via osmoregulation).
The observed changes are the result of a pathological response and not of a generalized stress response.

So what does the abnormal blood chemistry indicate? In short, it means the fish are suffering from infection, severe physical stress, and tissue damage. Their blood is incapable of distributing sufficient oxygen, the immune system is in overdrive and has become deficient, and the kidneys are failing.

What Level of Nitrates Should Be Allowed
Most of the scientific studies found do not take into consideration long-term life of the fish. They are designed to determine what level becomes lethal within a short amount of time or what levels impact reproduction. It is not surprising that the NASA test, specifically conducted to determine maximum long-term levels of nitrate, recommends keeping nitrates below 25ppm (7). A study on Fathead minnows attempting to determine when nitrate induced physiological impacts could be detected returned a result of 21ppm, with significant physiological differences being detected at 41.6ppm (6). A compiled review of prior testing conducted in Spain at the Universidad de Alcala suggests that the effects of nitrate toxicity in the most sensitive freshwater species can begin in concentrations as low as 2ppm and that long term exposure to nitrates in concentrations of 10ppm are known to adversely effect rainbow trout, chinook salmon, and cutthroat trout (1) (5). Only studies of channel catfish returned a result of what I would consider a high acceptable range, and that was at 90ppm (2).

Conclusion
Some are of the opinion that nitrate is perfectly safe at 80ppm or 100ppm. For some fish, this may be true. But I do not agree. Impacts may not be visible to the aquarist / aquaponist, but I believe a blood test would reveal a difference.
Nitrate, like ammonia, is a toxin. Fish bred in captivity far longer than our food fish (goldfish are a good example) are no more tolerant to ammonia & nitrate than are their wild counterparts. Adaptation to a toxin is an evolutionary step, requiring mutation, not something an organism can simply become accustomed to.

Of all of the articles, only one (pertaining to Channel Catfish (2)) identified anything above 25ppm as an acceptable safe level of nitrate for long term exposure and this study was geared more towards discerning short term effects, not long term.


Cheers
Toga

References

(1) Camargo, Julio A., 2004, Nitrate Toxicity to aquatic animals: a Review with new data for freshwater invertebrates, Chemosphere 58 (2005) 1255-1267
(2) Colt, J., Tchobanoglous, G., 1976. Evaluation of the short-term toxicity of nitrogenous compounds to channel catfish, Ictalurus punctatus. Aquaculture 8, 209–221
(3) Edwards. Thea M., 2006, Water Quality Influences Reproduction In Female Mosquitofish (Gambusia holbrookie) from Eight Florida Springs, Environment Health Perspectives, Vol 114, Supplement 1
(4) Hrubec, Terry C., Nitrate Toxicity: A Problem of Recirculating System, Aquatic Medicine Laboratiry, Virginia-Maryland Regional College Of Veterinary Medicine.
(5) Kincheloe, J.W., Wedemeyer, G.A., Koch, D.L., 1979. Toler- ance of developing salmonid eggs and fry to nitrate exposure. Bull. Environ. Contam. Toxicol. 23, 575–578
(6) Scott, G., Crunkilton, R.L., 2000. Acute and chronic toxicity of nitrate to fathead minnows (Pimephales promelas), Cerio- daphnia dubia and Daphnia magna. Environ. Toxicol. Chem. 19, 2918–2922
(7) Shimura, Ryuji, 2004, Nitrate Toxicity on Visceral Organs of Medaka Fish, Oryzias latipes : Aiming to Raise Fish From Egg To Egg in Space, Biological Sciences in Space, Vol. 18 No1 (2004):7-12
(8) DELLAMANO-OLIVEIRA, Maria José, SENNA, Pedro Américo Cabral and TANIGUCHI, Glória Massae. Limnological characteristics and seasonal changes in density and diversity of the phytoplanktonic community at the Caçó pond, Maranhão State, Brazil. Braz. arch. biol. technol. [online]. 2003, vol. 46, no. 4 [cited 2006-11-03], pp. 641-651.
(9) Mayorga, Emilio, 2002, Processing of Bioactive Elements in the Amazon River System, The Ecohydrology of South American Rivers and Wetlands. IAHS Special Publication no. 6, 2002
(10) Lewis, William M., 2006, Concentration and transport of dissolved and suspended substances in the Orinoco River, Biodegradation Volume 7, Number 3 / May, 1989, 0923-9820 (Print) 1572-9729 (Online)

[GaryD]

Hi Toga,

I agree that we need to think of nitrates as generally "less toxic" (when compared with ammonia and nitrite) rather than non-toxic.

We suffer for the generalised (and simplistic) "guidelines" that are often chucked around in aquaponics.

Having said that, I'm starting to develop a feeling that much of what is determined by researchers (particularly those that cater to commercial interests) can be treated with greater flexibility in a backyard aquaponics system than in a commercial unit.

I'm a strong advocate for the idea that any aquaponics system should be premised on a stand-alone recirculating aquaculture system(RAS).....but, once you connect a RAS to a growing system, you create a far more biologically complex environment than the "sterile" conditions that exist in most RAS......and, I suspect, some of the "rules" change.

Having said that, I believe that your central point is sound (that we do need to think differently about nitrates).....thanks for the information.

Gary

[Toga]

Hi Gary,

I also agree that aquaponics often can afford the user greater flexibility than the specific findings of researchers. Even some of their own conclusions can be 'open ended' according to application principals.

My post was prompted by posts & replies here and other forums that suggest nitrate exposure of 50-100-400ppm is not unsuitable for fish culture & production. One user posting that his system had been fish cycled for some months and stabilized at 80ppm nitrate... only to have 'unexplained' fish deaths 2, 3 & 4 months later. Not once have I read an opinion offering or suggesting that exposure to nitrate (or nitrite / ammonia) at some time in the past could be the reason for these 'unexplained' deaths.
I believe in many such cases an autopsy would confirm physiological impacts far from being 'unexplained', even to the extent that many months after deaths have stopped, if a 'healthy looking survivor' was sacrificed to autopsy many of these physiological impacts would still be self evident.

For 30 years I have be advocating to my customers that nitrate IS a toxin. A strong toxin that has far reaching health implications even if they have had no deaths.

The classic example is in the aquarium industry. The 2-4y/o oscar / barra / saratoga that dies for no apparent reason, living in 0ppm ammonia/nitrite & 40ppm nitrate... but 'the other fish are fine'. A 2-4year life span for a species that can live for 15-20 would strongly indicate all is not as well as it appears.

I try to ensure less than 20ppm nitrate for optimal fish health, though I do recognize that the 'ponics' side of aquaponics is greatly enhanced by equivalent or higher nitrate concentrations.

Cheers
Toga

[jobney]

I was wondering if like ammonia and nitrite in the biofilter, can't nitrate be brought into balance via the plants. If you could get the nitrate up to a detectable level yet not too high and attempt to hold it there you would at least be assured the plants requirements were not greater then the nitrate production. If it was on the way up some temporary plants like watercress might be used to keep the balance. I've always assumed everyone aimed for a balance at near zero. I'm glad this was brought up.

[GreenFin]

I've had the good fortune of not running into anybody who thinks nitrates are non toxic. Everyone I know fully understands that they are somewhat toxic relative to ammonia and nitrites (I say "somewhat" because in this context they are vastly less toxic than ammonia and nitrites).

The question of "How much nitrates are too much?" is inherently dependent on the type of fish and the goal of the APer. If you're trying to raise a species of fish that doesn't tolerate nitrates well, then nitrate levels matter far more than if you're growing an extremely hardy fish. Similarly, if you're wanting to raise fish to be 4+ years old, then nitrate levels matter far more than if you're raising fish that will be harvested when they're 6 months old.

The OP seems to be coming at this question from the perspective of someone who's wanting to raise sensitive fish for indefinitely long periods. That's valuable and perfectly relevant for a large number of us, but I don't want any newbies to think that it's necessary to keep the nitrates under 20ppm if they're growing extremely hardy fish that are only being raised up to harvest size.

For folks who are interested in growing hardy fish for short durations, I've read that UVI (University of the Virgin Islands) teaches that tilapia being grown out to market size can tolerate up to 500 ppm. Unfortunately, I don't know what time period this number refers to, i.e. whether they're saying the tilapia can handle, say, 450 ppm for the entire 6-8 month growout period without significant drawbacks, or whether they're only saying the tilapia can handle a spike to those nitrate levels for a shorter period like a week. (If someone knows, please share with us!)

For folks interested in growing more sensitive species, or in keeping individual animals alive and well for indefinitely long periods, running high nitrate levels seems like asking for trouble. I don't have any particular numbers to cite for any of the most relevant species of fish (it would be interesting to know "how much is too much" for various species over various time periods with respect to achieving various desired goals), but in the absence of such data I know that I myself would follow a general Toga-esque principle of "the less the better."

[Toga]

Hi,

I too have read many things from UVI, ie: Tilapia can tolerate 500ppm nitrate. They obviously have a world class system and monitoring measures in place. I would argue that anything less than a professional system, filtration & experienced staff that attempts raising fish at that level of nitrate would be doomed to failure.

Newbies on the 'sponge info up' stage of the learning curve also read this info, threads of 'my nitrate is 140ppm and fish are booming' and so on. They easily assume 50-80ppm nitrate is perfectly fine.
The example I used was a 'newbie' - "One user posting that his system had been fish cycled for some months and stabilized at 80ppm nitrate... only to have 'unexplained' fish deaths 2, 3 & 4 months later"... the fish he referred to are silver perch, considered to be a hardy fish, but not one reply that suggested perhaps nitrate poisoning could be responsible, even in his 'new' system at a steady 80ppm as he posted, go figure.

It would be a very interesting autopsy study & comparison of 4 tilapia batch's raised to market size... 1@ 500ppm nitrate 1@ 300ppm 1@ 100ppm & 1@ 5ppm.
Scientific research has proven nitrates are a toxin that can and do have devastating physiological impacts on various species at various concentrations & exposure. Also proven is that fish feed conversion & growth is higher, at lower nitrate levels.

I suppose the angle I was coming from is 2 fold -
1) To simply put it out there that because others quote their nitrate readings of 50ppm to 200/300 & beyond, with no fish deaths... does not necessarily mean the fish are healthy & happy.
2) On a broader more holistic note, would you prefer to eat battery chickens and eggs or free range chicken & eggs?

Personaly I would prefer to eat fish that did not suffer from or endure the effects of nitrate poisoning, at any level..... anemic blood & damage to the spleen, liver, and kidneys I do not find appetizing.

IMO the best info a newbie could get in regards to the fish would be to keep nitrates very low, Nitrate Is A Toxin and only with time & experience, the higher nitrate levels associated with higher stocking densities & feed rates will be easier to keep in check. Simply whacking in another grow bed does not address nitrate issues, though in time will help to minimize some nitrate related issues.

Most plants love nitrates - most fish do not.

Cheers
Toga

[Damon]

you can't really assign a number to this one... really you have to play it by ear...

you have the fish to consider, which that topic has been handled already... different fish can handle different amounts of nitrates, and really you'd only need to worry about the long term effects if you're planing to treat your fish tank like a show aquarium... people looking to sell the fish are only looking at keeping them in these conditions for a year tops...

but another thing to consider is nitrogen toxicity... too much nitrogen will make your plants look really really healthy... right up to the point where they basically crack and bleed out... so not only do you have to worry about the health of the fish, but the plants will also suffer with too much nitrogen intake.

[bigdaddy]

Thanks for posting that info Toga,

As you know I posted how my system and fish looked at running regulary 60-120 nitrates..Which was well and the fish looked really good to me...

My barra would of ranged from 6 months old to just over a year and the trout probably 4 months old...I did not have any intention of keeping the fish longer than a year..

If I was to stock Murray Cod or a slower growing fish then, by reading these posts I may well have some problems with unexplained deaths...

I have a couple of questions for you folk...

How can I tell if my fish is suffering from too high nitrates exposure? especially in the first six months...maybe I have not been looking for the proper indicators.

If we need to keep the water readings to a lower nitrates tolerance...what is that tolerance for particular species of fish and how do these tolerances change if they do in an RAS AP setup? (controlling the nitrates).

I do see what you are saying Damon but some rough figures would help me and the way I think.

Cheers.

[Ravnis]

I have actually noticed that plants do not do well at high nitrates levels. They show signs of retarded growth over 160 ppm. I have guessed this to be due to osmotic differences at the root level. The fish you listed: salmonids and fathead minnows happen to be some of the most sensitive fish I have read about. Nitrates require lots of dissolved oxygen in the water to keep from reverting back to nitrites. I question if the 02 levels were considered in these tests.

Reference #4 does not have 02 levels listed in the data. I have not gone through the other's.

Crustaceans are very sensitive to nitrate levels from every source I read and most recommendations for them was 5 ppm or lower.

I have noticed that my tilapia in the greenwater (algae filled tank) breed better than ones in my indoor RAS setup. The fact that nitrates are at undetectable levels in the greenwater system might explain the easier breeding.

[GaryD]

Hi,

Aside from direct impacts on fish health, high nitrate levels are problematic for other reasons including:

• When exposed to low dissolved oxygen levels, nitrates will be converted back to nitrites.
• High nitrate levels are believed by some researchers to be responsible for the muddy "off tastes" in some freshwater fish species.
• Elevated nitrate levels will produce nitric acid - which will lower your pH.

Gary