What Is A Safe Maximum Nitrate Level For Freshwater Fish?

Hi Folks,

I have been trying to establish a safe maximum nitrate level for freshwater fish. You'd think it would be easy to do just that but figures on the internet vary wildly - from 20ppm to several hundred ppm! I keep my tanks at less than 20ppm but that's partly to prevent algae outbreaks (along with phosphate levels).

Can anyone answer this question or point me to reliable sources of information on this topic?

Thanks in advance.

JPC

Someone who writes for Seriously Fish and is also a member of another forum has stated on that forum that the upper safe limit for nitrate is 20 ppm as measured by our test kits. He has given this source on that other forum https://www.researchgate.net/publication/8063535_Nitrate_Toxicity_to_Aquatic_Animals_A_Review_With_New_Data_for_Freshwater_Invertebrates
Note that the link uses the other way of measuring nitrate and the numbers in there should be multiplied by 4.43 to get them to the unit used by our test kits.

He also says that Dr Neale Monks recommends nitrate below 20 ppm.


Levels higher than this will shorten the lifespan of most fish and will cause illness in a few species - high nitrate has been cited as one cause of hole in the head in cichlids. High nitrate does not kill or harm fish quickly like ammonia and nitrite do.

Hi Sue,

Many thanks for the link and Dr Monks' recommendation. I have requested the full version of the linked scientific paper and will read it with interest when it arrives in my Inbox.

JPC

I must admit that I avoided this issue on my website and simply said the increase in nitrates between weekly water changes should be kept to less than 20ppm...
I also believe it depends on the fish species in question and I also keep cichlids and personally believe the link with hole in the head disease to be true...

The other factors to consider are tap water nitrate can legally be as high as 50ppm in the UK I believe so for some people 20ppm is hard to achieve (though there is always RO water this doesn't just strip out nitrates of course... and is another topic!). Further to this @jaypeecee i believe you are planning on having a planted tank which should mean nitrates are lower as the plants will consume them - so you could theoretically start the week with higher nitrates than ending the week. However, you must also consider that complete and/or macro plant fertilisers contain nitrates so you could well be adding them! I actually has a situation where I temporarily had very high nitrates as a result before I spotted it at my next set of water tests (like some others here I didn't do these very regularly nowadays but this has got me started again). It were the shrimp that seemed most suseptible most quickly to the high nitrates. I was dosing a complete liquid fertiliser at the recommended dosage but believe the slow growing nature of most of my plants meant it was not being used up quicker than it was being added. I now dose a combination of a complete fertiliser and a micros only fertiliser to keep my nitrates in check (I dose at levels i have tested to be effective - iron is a good indicator for micro ferts levels and I use nitrate of course as a measure for the macros).

I must admit that I avoided this issue on my website and simply said the increase in nitrates between weekly water changes should be kept to less than 20ppm...
I also believe it depends on the fish species in question and I also keep cichlids and personally believe the link with hole in the head disease to be true...

The other factors to consider are tap water nitrate can legally be as high as 50ppm in the UK I believe so for some people 20ppm is hard to achieve (though there is always RO water this doesn't just strip out nitrates of course... and is another topic!). Further to this @jaypeecee i believe you are planning on having a planted tank which should mean nitrates are lower as the plants will consume them - so you could theoretically start the week with higher nitrates than ending the week. However, you must also consider that complete and/or macro plant fertilisers contain nitrates so you could well be adding them! I actually has a situation where I temporarily had very high nitrates as a result before I spotted it at my next set of water tests (like some others here I didn't do these very regularly nowadays but this has got me started again). It were the shrimp that seemed most suseptible most quickly to the high nitrates. I was dosing a complete liquid fertiliser at the recommended dosage but believe the slow growing nature of most of my plants meant it was not being used up quicker than it was being added. I now dose a combination of a complete fertiliser and a micros only fertiliser to keep my nitrates in check (I dose at levels i have tested to be effective - iron is a good indicator for micro ferts levels and I use nitrate of course as a measure for the macros).

Hi Matt,

You are correct in saying that 50ppm nitrate is the UK maximum limit. But, if it's any consolation, the maximum value for my supplier (South East Water) in 2017 was 32ppm. Many aquatics manufacturers produce nitrate removers so achieving less than 20ppm should not be difficult. I use RO/DI water with added minerals so my starting point for nitrates is zero.

My main tank is planted but I choose to use root tabs as the primary source of macro and micro nutrients. I add liquid fertilizer once a week (at most). Then I add trace elements if the plants are beginning to show any deficiencies. Using this combination has eliminated any algae problems, which was an issue when relying only on liquid fertilizers. FWIW, I use Seachem's Flourish range of products. I also use CO2 injection. Seachem Flourish (Comprehensive) has only low N, P and K content. Regarding the measurement of iron as an indicator of the micro nutrients, I find it unreliable. That's because it's rapidly absorbed by the plants. I seem to remember that, after a couple of hours, there was virtually no iron left in the water column!

JPC

You can also mix with rainwater to reduce nitrates... especially if you have a plentiful supply.

Well, I decided to treat myself, the fish, and the amphibians.
Recently I bought myself a water change kit (similar to a Python).
Sue mentioned the Pozzani filters in another thread, so I had a look at them. I wouldn't be able to fit the filter cartridge under the sink, and this is a rented property, so couldn't start knocking things around. However, with the hose & connectors from the water change kit, I could attach the filter to the tap. In reviews on several sites a few people say that this filter also has an impact on pH, so I'll have to run some tests before I feel comfortable using the water in the tanks, but the though of getting water with 0ppm nitrates is quite exciting.
They also do a water hardness reduction filter....hmmmm, interesting. I'd have to be careful though as a lot of my gang were chosen as they were more suitable for harder water. May be useful instead of purchasing RO for the other tanks though.

Hi @jaypeecee ,

Thanks for the information. I planned to run some water off using the filter and test all parameters before considering how much to use in the tanks. Water here has nitrate levels of 40ppm, and a hardness of 17dH, so I was assuming that I could use a mix of filtered water and tap water to slowly adjust the water in my tanks.
Even my fish that are suitable for hard water shouldn't mind the levels dropping more towards the middle of their range, and I hope to be able to easily introduce the filtered water in the tanks where I use a RO/tap water mix.

I'm also hoping that I may be able to reach some sort of happy medium where I don't have to remineralise the water, even if it means only dropping my nitrates to 20ppm - that would be better than what I have at the moment. I'm looking forward to seeing what I can do with the water to provide my gang with a healthier environment, and hope they will be happy with the end result.

I'll probably also be using the filtered water in the kettle. The reduction in limescale issues should avoid me having to buy a new kettle quite so quickly. 

I attached the water hardness reducing filter cartridge to the nitrate removal cartridge, ran the water for a few minutes, then took a sample to test.

There was still a reduction in pH and KH levels, but not quite as much as with just the nitrite filter. The GH levels plummeted.    All other results seemed about the same as the previous test, including nitrate levels of 0.

I'm going to have a look at mixing filtered and tap water, and see what ratio I need to get the results I'm comfortable with. I'm guessing 50/50 (ish). Then I'll take several samples and test them multiple times to make sure the results are reproducible. This could take a while. 

@fcmf you are correct. 

I did test a small sample of a 50/50 mix of tap and filtered water. It was better than my tap water, but still not ideal, so I'll have a look at some more mixes tomorrow.

I've still got the Seachem De-Nitrate media to go into the filters, and am moving to TNC Lite (no nitrate) for the plant fertiiser. So rather than getting too concerned over the perfect mix of water types, I might just be grateful to be able to lower my nitrates, and be glad that every little change will help. 

I agree, it is a dramatic difference. 
I don't know why I was so amazed by the results, as all the reviews I'd read indicated that this is what I would see, but I'm always a tad dubious about these things until I see the results for myself.
Currently I have the filters in-line, so I'm going to separate them and look at mixing the water from them in a different ratio, and see what I can do with that.
The tiger barbs are due a clean & water change, so I plan to use some of the water with lower nitrates in that tank, as well as adding the Seachem De-nitrate media to their filter, and moving to the TNC Lite. If I just do it on this tank for now I'll have enough time for daily testing to keep an eye on things, and make sure nothing goes wrong.
I will keep an eye on the filter cartridge performance, as you suggest. With weekly pre-water change testing I usually run a tap water sample as a control because sometimes it's easier to look at colour difference rather than absolute values. It will be just as easy to also run a filter water sample at the same time. With the amount of tank samples I run, one more sample makes no difference.