Nitrates and A. ocellatus (oscar)

Posted in All About the Water

by Fishlab

This article has several intentions, first and foremost being a true understanding of the native environment of A. ocellatus and the captive environments we can reasonably provide. An unfortunate extension of this will ultimately be a rebuttal article to the current knowledge base on this website, although this is not intended as anything other than an independent peer review of available data. The opinions stated are my own, and should be reviewed to form your own independent one, there likely is no defined “right” or “wrong”.

Before getting into the meat of the discussion, I would like to state (and will reiterate at the end) that this is not a call for “dirty” water or an excuse to limit tank maintenance, but rather an opportunity to understand the needs/hardiness of this fish species and integrate a balance with our resources, time, etc.

*Links are provided via underlined numbers throughout the article*

Natural Environment

A current article focuses on nitrate levels, and draws conclusions based on scientific evidence. I do not wish to challenge the specific scientific evidence itself but rather the (mis)interpretation of such information and how it is applied to A. ocellatus. The article draws the conclusion that Oscars are “nitrate sensitive” and essentially suggests 10mg/l as a standard required for optimal health, although it is acknowledged that 20mg/l is the lowest referenced source available in other literature/resources.

I suppose “nitrate sensitive” can be interpreted by definition several ways. From an environmental standpoint, this is typically used to describe fish that are sensitive to any nitrate levels. These fish are typically from colder, fast moving bodies of water which have minute/no nitrate retention and may include but are not limited to trout, salmon, etc. Definition in aquaria is now applied to fish that possibly exhibit sensitivity in the lower end of the practical spectrum for fishkeeping, say (10-20mg/l/). This is where I believe A. ocellatus has inaccurately been placed when in fact this fish is hardy and capable of a wide variety of conditions.

If we focus on the body of A. ocellatus, we notice that it is large bodied, not particularly aerodynamic, which is typical for fish of the slow moving Amazon River basin/floodplains. Of particular attention, slow moving bodies of water are typically higher in nutrient content. This will be established in the upcoming paragraphs.

The Amazon River is incredibly dynamic and is often described as essentially three major water types, not a typical river that we often think of as similar from beginning to end. These water types include:

  • Whitewater is dirty white water created from the abundance of silt. This silt, brought all the way from the Andes is considered nutrient rich. There are few submerged plants as light has difficulty penetrating.
  • Clearwater tributaries flow through rock beds and thus have very little sediment and are thus nutrient poor.
  • Blackwater is stained by the tannins of decaying vegetation, these waters are nutrient poor with very little dissolved minerals.

Guess which type of water A. ocellatus is most often found? If you guessed the nutrient poor regions you would be incorrect, they are most often found in the whitewater regions of the Amazon Basin 2, 3, 4, .

While only making up 3-4% of the Amazon basin, A. ocellatus is known to thrive in the floodplains. For the purpose of this discussion, it is very important to understand what the floodplain is, how it exists, and what it does for the area.

Verbatim from the WWF (not wrestling ):

Why does flooding occur? As the snow melts in the Andes Mountains, water flows into the Amazon Basin and loads the Amazon River and its tributaries with copious amounts of water. But because the Amazon Basin is mostly flat and there is more water than the waterways can carry, the water spills outside the riverbanks and into the low-lying varzeas. Where flooding happens? Because of its vast area, not all parts of the Amazon Basin are subject to flooding at the same time. Generally, flooding occurs in the northern Amazon Basin (above the Equator) while floods are receding in the basin's southern part (below the Equator). Other parts can even experience two floods per year, which are different in intensity.

As the Amazon Basin floodwaters drain into the Atlantic Ocean, the water levels begin to fall and the forests “rise” again from the water. Wildlife reclaims its boundaries, and intense heat returns. The importance of the Amazon floodplain forest Flooding radically alters the forest landscape and is vital in dispersing sediment and in fertilizing the varzea. Heavy silt loads, primarily from the Andes, are a major factor in enriching soil for cultivation. 5. While soil enrichment can be of many things including nitrogen, phosphate compounds, etc. it should be noted that nitrates are specifically mined in the Western Andes (6 and can easily be assumed to be a major part of the nutrient rich silt that creates the whitewater environment and fertilizes the area. If there is any confusion, the river system floods with nutrient rich water once or twice a year which forms the floodplains, and then when the water recedes it leaves behind fertile soil.

Amazon River Dynamics and Evolved Oscar Hardiness

So, we've discussed the natural habitat of A. ocellatus, and I hope I have demonstrated that these waters change considerably during the flooding events of the wet season. Now, it may be argued that nitrate levels do not reach typical amounts in our aquariums, however, I find the 10mg/l argument to not be as important as the relevance that this fish has evolved to be able to adapt to its environment. One example of this is the ability of A. ocellatus to easily survive hypoxic (reduced oxygen) and even anoxic (absence of oxygen) conditions for a period of time. This occurs in the floodplains, as surface vegetation quickly grows and oxygen levels below decrease. A thorough study on the subject, which references both A. ocellatus and A. crassipinis, is available here 7 and 8, which indicates that these fish have a genetic ability to survive through metabolism suppression and if necessary anaerobic glycolysis.

O.k., so that isn't necessarily an indication of A. ocellatus and nitrogen, however, it does show we are dealing with a very strong fish. A study was completed though that compared oscars to another fish species of the same river. In the MarconMedian study 9, A. ocellatus was found to have an ammonia (NH3) tolerance (tolerant up to lethal concentration) 3X greater than C. macropoum.a fish that is also considered “ammonia tolerant”. I find it to be a logical conclusion that a fish tolerant of one form of nitrogen would also likely be tolerant to other forms of nitrogen as the fish is exposed to no high levels of any form in its natural environment.

Finally, A. ocellatus easily adapts to other environments. As I stated earlier, it typically inhabits the whitewater areas of the Amazon, however, it has also been found in the blackwater of the Rio Negro. Also, it has successfully been introduced to many different countries with different water qualities. A map available here 10 displays the different regions that the fish has been introduced and established. Not to mention, this fish was easily introduced to aquaria as an excellent beginner fish that will (and at times unfortunately) endure the learning curve. In addition, after 50+ years of aquaculture these fish have been far removed from nitrate free environments¦.and they readily made the jump into aquaculture of which true sensitive fish like discus absolutely did not.

Purpose and Conclusion

As stated earlier, the initiative for researching and writing this article was not to promote less than ideal conditions for fish. However, I do find it important to give aquarists, especially new ones that dominate the active membership here, realistic expectations about the fish in their care. While it is anyone's right to do daily or 100% water changes to keep nitrates close to 0mg/l, I think in our times of necessary responsibility for using resources and with the massive turnover in our hobby (which equals homeless fish) this needs to have a practical perspective so people can make their own decision. In my experience and opinion, I would propose levels of up to 25mg/l to be optimal water conditions, 25-40mg/l to be acceptable for long term health/growth, and 50mg/l + of nitrates to reflect degrading water quality that should be analyzed for improvement (appropriate feeding, gravel vac, etc.). I also feel the concentration on nitrates has led to a possible omission of a serious contributing 11. It is my belief that not providing a varied diet could very well be one of the leading contributing factors that suppresses the immune system and allows the HITH to manifest. It should be noted that “poor water quality”, amongst aquaculture/veterinarian standards when discussing this species, is considered in the 100mg/l+ range.

Enjoy your oscars, care for them responsibly and expect many, many years of rewarding fishkeeping.

*Note* The author has an 8 year old pair of A. ocellatus that have thrived in good water conditions ranging from 10-50mg/l depending on life circumstances (the “baby” years meant less time for maintenance but more time for observing) and currently is in the 20-30mg/l range before next water change and this is my preferred target. My entire system and opinions have largely emerged from observing this pair in various conditions.

References: The full content of the resources used are available in the links provided (1-11) and all material is the intellectual property of such sites/organizations. Information was interpreted by the author, no input/opinions regarding this article were provided by the references.

Fishlab Advice Team

Suseqeunt Forum Discussion

Re: Nitrates and A. ocellatus (oscar) by oscarsftw » Sun Aug 10, 2008 6:21 am

Nice write up, defiantly worth the read. 68 Gallon: Community; 125G Cichlid community coming soon. oscarsftw Microgeophagus ramirezi

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Re: Nitrates and A. ocellatus (oscar) by nitrodan » Sun Aug 10, 2008 8:51 am

As always, very informative. Thanks.My Tiger O vid:  nitrodan Symphysodon discus

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Re: Nitrates and A. ocellatus (oscar) by finis55 » Sun Aug 10, 2008 12:17 pm

That was a very good article, Thanks FL……….DaveThe trick to the trade and a pathway to prosperity won’t be found by those wallowing in the why, pointing fingers and placing blame, they’ll be conceived by those proactively positioned, readily prepared, steadfastly aware of what lays in wait -T. Harrison finis55 Market Maven

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Re: Nitrates and A. ocellatus (oscar) by screwsloose » Mon Aug 11, 2008 7:25 am

good job. love the links for references.75 qt 900gl reef born june 18th, 09 profile screwsloose Moderator

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Nitrates and A. ocellatus (oscar) by Herefishyfishyfishy » Mon Aug 11, 2008 8:44 am

Nice write up. Enter it as an Article if O'fish mag is going to have another go at it.-MIKE- Herefishyfishyfishy Moderator

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Re: Nitrates and A. ocellatus (oscar) by Polystigma » Mon Aug 11, 2008 4:44 pm


No, it is not odd to have seemingly disagreeing articles stickied in the same forum. It might lead to increased curiosity and who knows, someone might learn something.

Polystigma BanBot

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Re: Nitrates and A. ocellatus (oscar) by 0on » Mon Aug 11, 2008 5:54 pm

Good job FL, the writeup was very informative

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Re: Nitrates and A. ocellatus (oscar) by Al'Thor

Interesting. I definitely like to take in as many perspectives as I can.

Al'Thor Oscarfish Executive Chef

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Re: Nitrates and A. ocellatus (oscar) by finis55 » Mon Aug 11, 2008 10:53 pm

Nice choice for a sticky Poly……This is a very good read………

finis55 Market Maven

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Re: Nitrates and A. ocellatus (oscar) by Fishlab » Mon Aug 11, 2008 11:22 pm

Gracias Ted and I couldn't agree more. Data requires interpretation and none of us are infallible…heck, I would find it interesting if this stimulates a third article by someone else.

Fishlab Advice Team

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Re: Nitrates and A. ocellatus (oscar) by Ravyn_Smoke » Wed Aug 13, 2008 1:30 am

an interesting read…a lot of information to consider55 gallon two emperor 400s 6” Lutino Veil-Fin Oscar<Snowflake> Ravyn_Smoke Astronotus ocellatus

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Re: Nitrates and A. ocellatus (oscar) by wow_it_esploded » Wed Aug 13, 2008 1:32 am

Have been meaning to read this ever since it has been posted, but have not gotten around to it yet. Too tired to read it all right now, but will definitely read it later.

wow_it_esploded Astronotus ocellatus

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Re: Nitrates and A. ocellatus (oscar) by Kmuda » Tue Aug 26, 2008 12:03 pm

Fishlab and I obviously disagree, in some part, with this one. The problem is that there is no valid proof defining if either of us is right.

I hold fast to my argument. The studies that do exist, on nitrate toxicity in fish, provide indications that nitrate is toxic, over the long haul, in concentrations lower than most of us can maintain. The issue is that there has never been a study specifically related to A. ocellatus. However, studies have been accomplished on other species of fish, including Fathead Minnows. We know what the physiological impacts are to these fish and we know at what concentrations these impacts can be detected. Fathead Minnows are as “tolerant” to poor water quality (or perhaps even more so) than Oscars. So I take the information that IS available and apply it, as opposed to taking an absence of information and discarding it.

In addition, there is obviously a significant difference between the physiological cause and effects resulting from ammonia toxicity and those caused by nitrate toxicity. I feel much better taking known nitrate toxicity results, even those accumulated by studying other fish species, and applying them to an Oscar than I do making an assumption that because an Oscar is more tolerant of low oxygen conditions and ammonia than other fish that it is automatically more tolerant of nitrate.


Even the most “Nutrient Rich” locations of Amazon White Water have never recorded nitrate concentrations exceeding 2ppm, and that was during extreme drought. Granted, there are periods of time when Oscars get caught in pools following the end of the rainy season, but many of those become prey victims to predators in the area and, certainly, no study has ever been accomplished on how long Oscars survive in these pools and what the long term impacts are to those that do survive.


Which of us is right? Perhaps both of us. But I remain steadfast, without further evidence, that 20ppm nitrate is the top end at which our fish should be maintained, and the closer to 0ppm we get (and keep) the healthier our fish will be. This does not mean we panic when our nitrates exceed 20ppm… it means that we should not maintain levels of nitrate in excess of 20ppm for long periods of time. Jumps up to 20ppm, 40ppm, or 50ppm are to be expected. It's when those jumps become the baseline that I would expect nitrates to become an issue.

Kmuda Mad Scientist


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Re: Nitrates and A. ocellatus (oscar) by sandtiger » Tue Aug 26, 2008 12:11 pm

Darn it man, you know we need articles for the magazine! What you doing posting it here? sandtiger Senior Moderator

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Re: Nitrates and A. ocellatus (oscar) by Fishlab » Tue Aug 26, 2008 1:57 pm

The history of A. ocellatus in aquaria additionally supports my position. These fish readily adapted to, and quickly became, a popular ornamental fish that has been easily aquacultured. The reason for this is simple, they are incredibly adaptable to various water conditions due to their evolution in an area that changes dramatically by season. This is in stark contrast to a truly sensitive fish, the discus, that for years when first collected was considered very difficult to keep and equally difficult for aquaculture.


Once again, this doesn't mean I advocate “dirty” water conditions. What it does mean is that I believe that A. ocellatus is a perfect fish for new aquarists and a perfect fish in which people can execute an easy balance between observing their aquariums and with required maintenance and its costs (time, water, conditioners, etc.). I think a lot of the paranoia here, including being told that over 20ppm is going to cause HITH, is largely unfounded and un-necessary. A. ocellatus is an excellent candidate for the learning curve of new aquarists because of these adaptabilities and an equally good candidate for those that want a relatively easy fish to maintain and enjoy for a long, long time.


I've “been there done that” for 8 years with my fish, have seen various trends and products come and go, and am equally confident in my statements since my experience falls in line with my research.Help the economy, buy a house or refinance your mortgage….preferably in Schuylkill, Lebanon, or Lancaster county, Pennsylvania. Fishlab Advice Team

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Re: Nitrates and A. ocellatus (oscar) by Fishlab » Tue Aug 26, 2008 2:01 pm

sandtiger wrote: Darn it man, you know we need articles for the magazine! What you doing posting it here?

To be honest, I thought the magazine was idle until Rush contacted me. Should this be formatted for it, or since it exists here is it “unavailabe”. I've got other topics in my head, although I might just shoot you some saltwater ones since they're already finished.

Fishlab Advice Team

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Re: Nitrates and A. ocellatus (oscar) by sandtiger » Tue Aug 26, 2008 2:44 pm

I was just kidding around but we will need articles for the magazine so if you few like churning out a few it'll be appreciated. This one can remain here. sandtiger Senior Moderator

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Re: Nitrates and A. ocellatus (oscar) by Kmuda

Fishlab, let me begin by stating… welcome on-board. Another point of view is always welcome. I also welcome any argument that makes me think, and I hope you do not take any of my counter argument as personal.


On this subject, I believe we can agree to disagree. I cannot draw any comparison between ammonia toxicity and nitrate toxicity. The only common factor is that both are nitrogen-based pollutants, but this is like saying that carbon dioxide toxicity is the same as carbon monoxide toxicity. Each affects the body in a different way.


We know that ammonia is toxic to all vertebrates. It is believed this toxicity, and subsequent lethality, results from cell death in the central nervous system (elevated NH4+ (Ammonia Ion) displaces K+ (potassium ion) and depolarizes neurons, causing activation of NMDA (N-methyl-D-aspartic acid) type glutamate receptor, which leads to an influx of excessive Ca2+ (calcium) and subsequent cell death in the central nervous system).


Studies associated with Nitrate toxicity do not indicate the same physiological impacts. There has been no evidence, as far as I am aware, that Nitrate toxicity has any impact on the central nervous system. While ammonia appears to impact the central nervous system, excessive nitrates appear to impact the kidneys, liver, spleen, and heart. There can be some level of comparison between nitrIte poisoning and nitrAte poisoning, but not between ammonia and nitrate.


Obviously, ammonia kills quickly, so is much easier to study, and as a result is somewhat more understood. The effects of nitrates, especially in the low concentrations we are attempting to maintain, are much less understood. In fact, they are totally NOT understood. It has never been studied, and probably never will be, because such a study would involve years. So we are left with a minimum number of studies, primarily conducted for commercial fisheries, to determine at what levels nitrates can be maintained without affecting health or growth (which would affect the bottom line).


So… the numbers from these studies is what we are left with, and almost universally, these numbers indicate between 20ppm and 25ppm as the maximum level for long-term exposure. I choose to apply them; regardless of what species the studies focused on, as opposed to discarding them because they do not apply specifically to A. ocellatus. In my opinion, the availability of information trumps an absence of information. The physiology of a Fathead minnow is close enough to an Oscar (heck, they both have the same internal organs, interact with their environment in the same way, and have the same internal body chemistry) that the use of these nitrate toxicity numbers is preferable to not having any information.


Your referenced study indicates a 96 Hour LC50 (half the animals studied in a 96 hour period died) level of 2.01ppm of ammonium chloride. A similar study on Fathead Minnows indicates a 48 Hour LC50 level of 3.4ppm ammonium chloride. Close enough of a comparison that I would consider a Fathead Minnow equally tolerant, if not more so, to ammonia concentrations as A. ocellatus. Certainly, no drastic difference exists. If a Fathead Minnow is equally tolerant of ammonia and the effects of nitrate concentrations can be detected in Fathead minnows at 21ppm, then I consider the transfer of those numbers to. A. ocellatus justified. When you add that other studies, pertaining to other species of fish, also come to the 20ppm-25ppm conclusion, I consider the 20ppm number applicable to oscars. That, and for some reason I do not understand, Fathead minnows appear to be the scientifically accepted “generic” test critter for these types of studies.


I also would not consider your experience of 8 years in keeping your Oscars as a valid indicator of either success or failure with nitrate levels. I would consider 8 years an age not uncommon when keeping Oscars, even for people who take much less care than we do. We know that Oscars can live at least 10-15 years (and I think we can get to, or exceed, 20), we also know that the effects of nitrates accumulate over time in that we have clear indications that the MCL for nitrates decrease as time periods increase. We also have the experiences that illnesses resulting from significantly excessive nitrates may not manifest themselves for 2-5 years of long term exposure.


I will take my 18-year-old (edit Kmuda, now over 20 year old) Kissing Gourami as an indicator that abnormally low nitrate levels, maintained over a long period of time, can result in long life.


I do recommend keeping Oscars in as low of nitrate concentrations as possible, and strive to keep my fish at 10ppm or less. I do not consider the 10ppm a necessity for the average fish keeper, but I am trying to get my fish to the 15-20+ year age.


I would not be alarmed with another aquarist's tank that approaches 40ppm within a week or so, as long as a majority of that week the fish is living in concentrations that are below 20ppm and the aquarist is diligent enough with water changes that the fish spends more time in concentrations below 20ppm than it does in concentrations that exceed it. I, personally, would be alarmed if MY tank exceeded 20ppm. But I understand that not everyone is as dedicated as I am to this process or as interested in the limited end results that dedication, over an extended period of time, may produce (If I manage to have an Oscar live 20 years, big deal¦ heck, my fish may outlive me). Of course, the easy answer to this “effort dilemma” is a very large, under stocked, tank.


Finally, another point of potential disagreement, relates to Oscars being an acceptable fish for beginners. I do not necessarily disagree with this conclusion. My potential point of contention is that you should be sure that fish is a “hobby” you care to be involved in, willing to provide sufficient tank size and effort (and by effort I mean weekly water changes, monthly filter cleanings, etc), before you get an Oscar. An Oscar is going to require more effort, expense, and is a longer-term commitment, than a community tank, or something along the lines of Angelfish or Convicts. It is only for this reason that I would question the recommendation of Oscars as a “beginner” fish.

Kmuda Mad Scientist

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Re: Nitrates and A. ocellatus (oscar) by Fishlab » Tue Aug 26, 2008 7:54 pm

1)If the physiology of fish was “equal”, they would all react equally to a given substance…..they don't. Heck, people have the same regulatory systems and organs but we react greatly in exposure to substances.


2)I firmly believe misinterpreting data can be equally as erroneous as an absence of data, I would even be willing to state it can be more dangerous. This is proven by the medical field frequently.


3)If my 8 years of oscar keeping is inconclusive, the same can be said for your success with your gourami. If your experience is reflective, then 8 problem free years with only one instance of medication is also a good sign of mine. I also would not confuse water changes with dedication. Anyone that knows me can attest there is not a more dedicated aquarist. However, I also acknowledge the law of diminishing returns and do not believe or have any practical observation that 0ppm has any benefit to outweigh the additional time and cost….and neither do you. Nitrates is but one factor in longevity amongst other simple facts like water temperature, breeding history, etc. Spawning alone greatly impacts lifespans.

4)Expecting all fish keepers to have complete knowledge of the fish they want to keep beforehand is unrealistic. Heck, it's important to remember that most people assume an LFS is a good source of information.


I'm pleased with my article and outlook, not much more comment required.

Fishlab Advice Team

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Re: Nitrates and A. ocellatus (oscar) by Big Vine » Tue Aug 26, 2008 10:42 pm

I'm very impressed with both standpoints given.

Well done, gentlemen. BV75 gal. - 11” tiger oscar Big Vine Astronotus ocellatus

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Re: Nitrates and A. ocellatus (oscar) by Fishlab » Wed Aug 27, 2008 7:37 am

I agree, and did notice in rebuttals I forgot to mention that indeed neither of us have defined proof and never will. Much of this comes down to a different perspective in philosophy…..and obviously a different interpretation of data. Folks can gleam plenty of info for whichever philosophy appeals to them.

Fishlab Advice Team

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Re: Nitrates and A. ocellatus (oscar) by Phibenet » Wed Jan 27, 2010 10:34 pm

Nice write up.