FAQs on Marine Freshwater Quality involving
Related Articles: Nitrates in Freshwater Aquariums, Establishing Cycling, Freshwater Filtration, Know Your Filter Media, A
Concise Guide to Your Options by Neale Monks,
Setting up a Freshwater
Aquarium, Tips for
Beginners, Water Quality
and Freshwater Aquariums,
Related FAQs: Nitrates
1, & FAQs on FW Nitrates: Importance, Science,
Measure, Sources, Control, Chemical
Filtrants, Troubleshooting/Fixing, & Ammonia, FW
Nitrites, Biological Filtration, Freshwater Nutrient Cycling, Establishing Cycling 1,
Stress from Water Change Worse than having 20ppm Nitrates in
new Tank? 10/11/17
<Helen et al.>
I'm new to fish-keeping but have done a lot of research and hope I have
got my aquarium off to a good start. I cycled it for six weeks
using a fishless cycle with fish food for the ammonia source. I
have a 20 gallon freshwater tank with two male Dalmatian mollies, 4
female Dalmatian mollies and two female gold mollies. All but
one are less than 1 inch in size, so quite young. I know I may need to
move them to a bigger tank in the future. They have been in the tank for
eight days and I did a 25% water change four days ago and plan on doing
one every week.
<A good interval and percentage. Best to store the new water in advance
of your weekly changes; do whatever you intend to supplement (add
salt/s, alkalinity...) ahead of time>
I have been testing the water everyday using the API water test kit.
Today's readings were: Ammonia:0; Nitrites: 0; Nitrates: 20; PH 7.6.
Temp is 78.4F.
I know mollies are particularly sensitive to nitrates, so I would like
to do another 25% water change today to get the nitrates down. However,
I have also read that all fish are sensitive to changes in water
chemistry so I
am wondering what would be most harmful to the fish - having the
nitrates at 20ppm or doing a 25% water change.
<You are right to be concerned here. As you hint/state there are
trade-offs in doing too frequent/serial dilutions, and just tolerating
nitrogenous et al. accumulation>
I would also like to add some aquarium salt to the water but am again
hesitant to change the water chemistry too
much when they have only been in the tank for eight days.
<I WOULD go ahead with the salt addition/s... some every day. This will
also reduce the Nitrate toxicity>
Your advice is much appreciated!
<And gladly rendered. Bob Fenner>
For Neale Monks and Bob Fenner
Hello again friends,
I’d appreciate your comments and reasoning about a question involving lowering
nitrate. I will start by saying that I understand that nitrate like ammonia and
nitrite is toxic to fish, though at much different levels/exposures and
depending upon species or age of the fish (fry being more susceptible) [please
correct me if I am incorrect here or anywhere else].
<This is correct; though the mechanisms, pathways if you will, for
poisoning/toxicity of these nitrogenous compounds are different. NO3 in
particular can be accommodated; i.e. much higher concentrations can be tolerated
with long exposure>
Consequently, if one discovers nitrate levels in an aquarium are high, say 160
ppm [I am using an actual case from my work on TFF, and this is a stable state
not something sudden], immediately reducing the nitrate to safe levels (under 20
ppm) is not in itself going to harm fish.
I have been challenged on this, with the suggestion that the nitrate should be
lowered gradually over days or weeks, similar to other adjustments.
<Mmm; no. Immediately lowering NO3 concentration is advised>
The idea apparently is that “old tank syndrome” is dangerous and rapid changes
can be fatal; but I would respond that the danger with this is due more to pH,
and ammonia being ammonium in acidic water and the sudden change to basic pH (pH
shock, plus ammonium converting to ammonia) is the problem, not nitrates
decreasing. My argument is that nitrate is not like other adjustments (GH, pH,
temperature, or whatever) and being toxic the sooner it is lowered the better.
I maintain that any toxin in the water, be it ammonia, nitrite, very high
nitrate, substances released from wood or rock that are detrimentally affecting
fish, etc, are best corrected rapidly via significant water changes.
Comments please, with thanks.
<Will ask Neale for his separate response here. Bob Fenner>
Re: For Neale Monks and Bob Fenner: Nitrate
I don’t have any real insight into this. But I do wonder if there are
differences between species and when comparing marine with freshwater fish. My
point being that generalist freshwater fish are able to handle bigger water
chemistry changes than more specialist species (or most marines) given they’d be
exposed to such in the wild. For example, the pH of a pond can vary between
around 7 to as high as 9 once photosynthesis kicks in and dissolved CO2 is used
<A useful point/speculation. I do think there are differences between
salt/fresh, young/old, acclimated and not species, specimens. Have been to
public aquariums that fed huge amounts of food to very large animals... that had
thousands of PPM of NO3>
I’ve read before that the idea we can meaningfully acclimate fish to slight pH
changes is actually erroneous anyway. The “float them in a bag for an hour” or
“drip water into a bucket for an hour” approaches sound good, but supposedly the
actual physiology works far more slowly than this. So for fish to actually adapt
their blood chemistry (or whatever) actually takes far longer, and what we’re
really dealing with is the degree to which fish can tolerate abrupt changes
(i.e., shock) and then slowly adjust across days or weeks. Does this sound
familiar to either of you?
<Yes; it does>
I do believe, Byron, that there’s a hierarchy of stress factors, and sometimes
to minimise a severe stress (such as nitrite, ammonia or extremes of
temperature) you may have to increase a mild stress (such as small pH or
hardness changes) simply through doing water changes. Of course the standard
advice should remain that water changes need to be made with water as similar to
the conditions in the tank as practical.
So far as I know, nitrate toxicity hasn’t really been studied across a wide
range of ornamental freshwater fish, but experimentally with things like
goldfish you really do need quite high levels (100+ mg/l) to cause immediate
health issues. In such situations, I think doing moderate water changes across a
few days, rather than one giant water change, might be safer in terms of
minimising sudden pH, temperature or hardness changes. But that said, if the new
water was similar enough to the old, doing 90% water changes has been
demonstrated to be perfectly safe in and of itself.
Anyway, keep me posted with what you learn!
<And you, BobF>
Re: For Neale Monks and Bob Fenner
Thanks Neale and Bob.
So what I take from both of you is that with the proviso that parameters (GH,
KH, pH , temperature) are close enough to be called the same, a large water
change to reduce nitrate from 160 ppm down to 10 or 20 ppm is not going to harm
the fish, and is more advisable than doing smaller changes over weeks. I will
assume my understanding is correct unless you say different.
<This is a good summation>
I do appreciate the benefit of your experience and knowledge on these issues.
Re: For Neale Monks and Bob Fenner /Neale
I would 100% agree with this.
Triage of any kind is about balancing the big dangers against the minor
0 ammonia, 0 nitrites, 160 ppm nitrates during fishless cycle
I am trying to cycle my 10 gallon tank (for a Betta) using the fishless
ammonia method. It's been 6 weeks and ammonia and nitrites
spiked and the fell to zero.
My questions are: why are the nitrates so high,
do I continue to dose with ammonia (I have stopped)
will the nitrates drop eventually like the ammonia and No3 ?
<Mmm; slowly; yes; but better to either add some live plants... or do a
significant water change... half the water, halve the NO3>
I have just done a 90% water change and nitrates are at 40 to 80 ppm.
The tank has a HOB filter, a sponge filter with airstone, a heater and a
UV sterilizer. The pH is 7.4, the tap water is hard (and 10-20 ppm
There are some (3) plants and the roots of a philodendron in the tank. I
plan on adding more plants. What do I need to do?
<Really; just be patient; time going by... Monitor/test every few
days... NO3 will "go down".>
<Welcome. Bob Fenner>
Re: 0 ammonia, 0 nitrites, 160 ppm nitrates during fishless cycle
Thank you Bob Fenner!, should I be feeding the bacteria with fish food
until the nitrates come down (just bought 4 more plants)?
<Yes; but really... "just a pinch" or a single flake or two per day.
Takes very little to sustain a nitrifying population>
<Again welcome. BobF>
Re: 0 ammonia, 0 nitrites, 160 ppm nitrates during fishless cycle
Great, thanks again for the info.
<Cheers Eve. B>
For Neale Monks, Nitrates 1/8/16
Hello Neale, and my best wishes for the New Year to you and yours.
I will be emailing separately my observations on the flashing/bacterial issue
you helped me resolve (thank you so very much) but now I would like to discuss
nitrates a bit if you would be so kind. I have frequently read your advice on
keeping nitrates below 20 ppm, and naturally I recommend the same when I respond
to members of the forum I am now on. I am being questioned on this, so I would
like to understand it better.
First thing to get sorted is the unit being used. The scientific community tends
to use NO3 N-n, whereas most hobby test kits use total nitrates. I understand
the conversion factor is 4.43, so for example the US EPA limit of 10 ppm
allowable nitrate in drinking water which is NO3 N-n would equate to 44.3 ppm
NO3 with our test kits. When you are recommending 20ppm as max for nitrates, is
this the hobby test kit unit (I assume so)?
<Correct. No point citing something people can't (easily) measure at home.
In fact it doesn't usually matter what numbers you choose to use. Most of the
nitrate kits I've seen will have some sort of card with them, and that card will
be have on it a scale made up of a few coloured patches. The API one for example
has seven coloured patches, from yellow (low) to red-brown (high). So long as
you aim to keep nitrates closer to the low end of that scale where sensitive
fish are being kept, and certainly below the medium colour/number on that scale,
you're laughing. Actually knowing what the numbers are is not important. So for
that API kit, yellow or orange are fine, red not good for sensitive species like
dwarf cichlids, and red-brown probably too much, long term, for anything, at the
very least a triggering factor for algae. Make sense?>
To the nitrates, then. Natural habitat waters of all of our fish (so far as I
know) have nitrates so low it would probably be impossible to measure them with
<More or less, yes.>
In the fish, nitrate will act much like nitrite, making it more difficult for
the blood to carry oxygen.
<So they say.>
I have come across studies, admittedly on mainly commercial fish and not
ornamental, suggesting nitrate levels of 2 to 4 ppm NO3 N-N would affect the
development of fry, and many fish and invertebrates will have difficulty with
nitrate at 10 ppm NO3 N-N. This study is here:
You have mentioned cichlids likely being affected at levels above 20 ppm, and on
the cichlid site it is suggested that nitrate above this may be the true cause
of Malawi Bloat.
<Correct. That said, nitrate is complicated. The lethal level of nitrate for
some fish experimented on (in labs) seems to be high, supposedly 100 mg/l or
more. On the other hand, scientists have only tested a very few (big) species,
farmed trout for example, and often their experiments are shorter term things,
like how many of the fish die across, say, a week or a month. We're keeping a
hundred different species often for years if not decades, so our experiences
(and expectations) are different.>
The above is just so you have an idea of where I am with this issue. I guess at
this point, I would be interested very much in your thoughts on this, and any
evidence, studies, etc., to support our position [I have certainly taken yours ]
on keeping nitrates low.
<It's complex. Nitrate isn't something I worry about too much keeping the sorts
of fish I like to keep, such as catfish and freshwater livebearers.
Provided other parameters are good, fast-growing plants keep nitrate levels low
enough risk of toxicity isn't an issue, and water changes can be carried out
as/when required. But if I was keeping species like Dwarf Cichlids or Mollies
(in freshwater) that are known to be sensitive to nitrate, in the sense that
high nitrate has been associated with disease (bloat, shimmies, etc.) than I'd
make more of an effort to monitor nitrate and pre-empt any high nitrate
situations using low stocking, low food input, and frequent water changes. Most
freshwater aquarists can, I think,
ignore nitrate unless they're (a) struggling to keep a possibly sensitive
species; or (b) dealing with an algae problem.>
As always, looking forward to your wisdom, and with sincere appreciation.
<Hope this helps; by no means an expert! Cheers, Neale.>
Re: For Neale Monks, Nitrates 1/8/16
This is great, thank you Neale. Now I have some follow-up, concerning the
effects/symptoms of nitrate on fish.
Many if not all of your answers to questions on PFK where nitrates are mentioned
as being high involve lowering to no more than 20 ppm. From this I would assume
that the effect of nitrate is much like so many things--a source of stress,
weakening the fish, opening up opportunities for more serious problems.
<Correct, so far as I can tell. Nitrate isn't immediately toxic like
ammonia or nitrite. On the other hand, because high nitrate levels often go
hand-in-hand with things like overstocking and infrequent water changes, it's
hard to pick out any problems nitrate is causing from things like lack of oxygen
and background acidification cause by nitrate and phosphate accumulation. Tanks
with high nitrate levels tend to be neglected tanks, in the sense that the fish
keeper has too many fish in them and does too few water changes. So there can be
all sorts of reasons fish in those tanks are stressed, not just the nitrate.
One obvious that I assume would occur would be a shortened lifespan from the
stress if nothing else. But are there any signs along the way that nitrate may
be causing issues?
<None that I'm aware of, but some diseases have been associated with high
nitrate level, such as Hexamita and HITH/HLLE in cichlids.>
A member on the forum today mentioned his fish being "fine" with 80 ppm nitrate
(which is way into the red on the API card you mentioned) but his Firemouths
remained quite pale, and he wondered if this was due to the nitrates. I would
think this likely, do you agree?
<It's certainly a possibility. But I'd also observe that Firemouths are widely
kept badly. Though territorial, they're bluffers, not fighters, and do badly
with genuine fighting cichlids (pretty much all the Central Americans beyond
Rainbow Cichlids) and are really best kept on their own with dissimilar
tankmates (catfish, loaches, characins, etc.). They're also sand-sifters, so a
tank with gravel would be wrong. Finally, like all cichlids, their colours will
become paler if they're exposed to bright light from above and/or below. A dark,
shady aquarium is better.>
Balloon belly mollies... Not... NO3
I have a 20 gal tank 6 moolies,35gal Eheim filter, I do water change
10% every week. My nitrates are up over 40+.
<Too high by at least twice>
I have used nite zorb,it did nothing, I have a RO/DI system. What can I
do about the nitrates? Thanks
and the linked files above. Bob Fenner>
Nitrates, FW 3/1/11
I have a 55g setup containing a Black Ghost Knife, an Oscar, and a
<Far too many fish in very little water, plus the Black Ghost is
sensitive to low oxygen concentration, and the Oscar sensitive to
The tank has been running for a little over a year now and the fish are
producing more wastes resulting in higher nitrate levels. I do a weekly
water chance to bring the nitrates to under 10 ppm but by the next week
they are high again. I would ultimately like to get a 125g for these
but in my current living and financial state that is not possible.
<Very short term I'd rehome the Pleco -- adds nothing to this
system. Algae better cleaned by hand, and all the Pleco does is dump
solid waste into the filter, reducing filtration, while cranking out
lots of ammonia.>
My short term solution would be to get another filter, maybe a
Marineland Emperor because of the bio wheel.
<Won't have any effect on nitrate.>
Would the addition of another filter prevent nitrates from accumulating
so much between water changes?
<No. Biological filters *make* nitrate. The amount of nitrate is
proportional to the number/size of the fish. To remove nitrate you have
three main options: fewer fish, more water changes, and/or less food.
Denitrification in aquaria is difficult to do without considerable
expense and engineering, and plants, while excellent nitrate removers,
aren't going to make much difference in a tank this size/stocking
density without very strong lighting and perhaps an external sump with
plants in there as well.>
Also, my Plecostomus has hardly grown the entire time I've had him
while my Oscar that I bought the same day has gotten huge. I realize
the Oscar grows faster and larger but the Pleco hasn't even grown 2
inches in a year.
<Not getting enough of the right foods?>
I was thinking of adding some of the plant food that I use in my 29g
planted African tank (Leaf Zone) to stimulate algae growth so the Pleco
can have more food available. Thank you for your advice and
<More algae = more food = more nitrate. Hardly what you
<Hope this helps. Cheers, Neale.>
Re: Nitrates 3/1/11
Thanks, it does help. I just can't get a large aquarium yet, as
much as I'd like to. I had considered rehoming the Pleco. I have 2
pumps in the tank to circulate water and detritus. The Ghost Knife
loves hanging out right in front of one of them.
<Yes; they inhabit pools and rapids in the wild, so prefer (need,
long term) lots of water current.>
I don't trust the denitrifying chemicals or units plus they are
Both large fish are almost fully grown so hopefully it won't get
much worse. If I got rid of the Pleco I feel I should be okay with
weekly water changes.
<Possibly, but 55 gallons is really not much for an Oscar and a BGK;
75 gallons better, but 100+ gallons realistic for long term
They both seem to get along fine given the amount of water that they
<Indeed, but they will both grow, and both are prone to slowly
developing issues that aren't obvious. Oscars develop
Hole-in-the-Head and Hexamita infections when exposed to less than
perfect conditions. May takes years for it to become obvious.>
I got them about a week apart and I've never seen them fight.
Actually the BGK and Pleco fight more, most likely because they occupy
the same space.
<Likely so, and both are territorial.>
Thank you for your insight
<Glad to help. Cheers, Neale.>