Controlling Nitrates in a Saltwater Aquarium

Nitrates can kill every fish and coral you have in your tank if they aren’t kept in check.

The good news is that keeping them in check requires patience and using several nitrate reducing tools that I cover in this episode of Mr. Saltwater Tank TV.

Links in this video: Go Plus One: Bump it up a notch

NOTE: If you want to leave a comment, click on the “comment” link below the video.

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Comments for this article (84)

  • Bob says:

    Hi Mark, I have a 60L nano tank with live rock, which has been running for 3 weeks now, my question is other that replacing evaporated water, do I have to do a water change, got all the books (well, a lot of them) they tell me a lot of things, confused, I know you can help.
    thanks (newbie) Bob.

  • Alicia says:

    I would. I’m not sure if it makes a difference if it’s still cycling, but I know Mr. SaltwaterTank recommends 5% weekly, or 10% bi-weekly. I would do it just to give my tank a healthy helping hand.

  • Jeremy says:

    Mr Saltwater Tank,

    Great post. Love your website. Hope you don’t mind me saying but I couldn’t help but notice you have some mean looking lid styes there. If you want to avoid those in the future, incorporate more fish in your diet. Salmon, tuna, and mackerel are very high in Omega 3.

    I have a 29 gallon biocube and recently added a sump/refugium. I added chaeto about a two months ago and it grew like crazy. I pruned it back once and now have noticed it has not grown much at all. Is it possible nitrates are too low now to support additional growth of the chaeto?

  • Tony says:

    Great post. Useful & informative info. Thanks Mark!

  • Mike says:

    Mark – First, thanks for the tips and videos. They are extremely helpful.

    My household is new to saltwater tanking, so I apologize up front for the long post. Perhaps someone else can learn from our mistakes…

    We just love the saltwater tank, and already want more, bigger, everything. We have a used 25gal that we started with (yes, I now know. Too small). The tank setup is 4 months old. We don’t have a lot of livestock (a spotted puffer, two clowns, and a couple of hermit crabs), and about 20 lbs of live rock. Things have been good in the tank except for the past 3 weeks, where we have very high, persistent nitrates (80-160ppm), and the gradual appearance of a brownish, or orange algae film on the glass, and dark red and even purple stuff on several places on the rock (which I think might be cyanobacteria). We clean this stuff off during water changes, but it comes back of course.

    We have been doing water changes weekly (5 gal each week), and have bumped to 10 gal changes. It knocks the nitrates down, but they come back. I don’t think we are overfeeding. I am at a loss thus far what the source of the nitrates are – why they are so high so fast…can three fish and some left-over food generate this much nitrate? (ammonia and nitrites have been testing zero for a couple of months, BTW). The fish seem OK, so far.

    We don’t use a skimmer (we have a HOB Aqeon carbon filter), nor do we have a refugium. I am thinking at this point we need some sort of macroalgae and/or a skimmer to get the nitrates under control. I don’t know how long it will be before we could increase the tank size, and related equipment, so I really want to understand how best to manage this problem with the present tank, if possible.

    Thanks for any suggestions!

  • saltwater tank boy says:

    Hey Mrsaltwatertank can you do a video on carbon dosing, most likely bio pellets. I was also wondering if you would be able to do a video more on kalkwasser.

  • Tempdoom says:

    I kept my nitrates down by waiting 2 or so months before adding another fish, and not having many. i have a clown, six line wrasse and a TANG in my 28gal nano cube, and i do not have measurable nitrates. i have never done a water change and my water is pristine.

  • While your nitrate may be zero, you should consider moving your tank into a more properly sized (55g +) system. Tangs need lots of swimming room to feel comfortable and a nano cube is simply too small.

  • menet says:

    i heard about this product called aquaripure can you please do a review on it they have there own website im just not sure it works i am considering one for my tank (fish only)

  • menet…save your money and don’t buy the aquaripure. If you set your tank up correctly, you don’t need a nitrate reactor.

  • Mike says:

    Mark, I see so many different answers on this. I have a 2″ sand bed and believe its why I’ve been chasing nitrates. I have larger skimmer, added tons of live rock, feed lighter than I ever thought I would, run bio-Pellets, DeNitrate, sand sifting clean up crews, and so on. This sand was in the tank when I moved in, not sure how long its been there, probably 8 years or so & the tank was very neglected. Do you think it may be a good idea for me to syphon the sand bed? I was thinking every 2 weeks during water change do a small area like 1/8 of the sand bed. And ONLY the top layer like 1/4 inch. I totally get why people say its not a good idea, thats why I’ve never done it, but now I’m spending more time considering it. Thoughts?

  • Tom De Meulder says:

    Mark, I have a rather urgent question regarding nitrates.
    Yesterday i measured my nitrates and got around 50PPM.
    I freaked out! I think lately i overfed my fish a little.
    I reduced it from 2 times a day to only one time a little bit every day.
    My tank runs now for about 4 and an half months. I started with fresh live rock and had it running in for 8 weeks before adding 1 fish.
    My tank is 270 liters and have a turboflotor blue 1000 skimmer.
    How do you suggest getting rid of the nitrates? It is still the same water as with the startup. Yesterday i did a 25 liter change but it didn’t really knock the nitrates down. Do you think my tank is biologically mature enough do do some large water changes? The rockwork is pretty open and i did not have any dead fish yet. Until 2 weeks ago i did not have nitrates above 10PPM. Thanks in advance!

  • Tom…you’ll need to do a 50% water change or better to make a dent in your nitrates. And, make sure the test kit you are using is one I recommend. A poor test kit can lead you to inaccurate conclusion. Here’s where to get my test kit list.

  • Tom De Meulder says:

    Mark, thanks for the swift reply! Tomorrow i will have a look on how to get 150 liters here. I looked into the test chart you have and i use the same tests from salifert. I think it is rather hard to compare the color (specially on the nitrate test). The booklet says you have to compare the colors seen from above on the white surface next to the pink colored boxes. The fish store keeper tells me it is easier to compare if the test vial is on top of the colored boxes. Any advise on reading the values in the right way?

  • Calvin O. says:

    Hey Mark! How’s it going? I look forward to your videos every week! Anyways, here is my question: I have a 40G tall Mixed reef which is beautiful. Now I am working on a project in another room specifically designed for Clown fish. I would love to start breeding them. They are my favorite fish and I have done a ton of research on breeding. I have several rare and interesting pairs. Ok, so my question is I am setting up 4 identical tanks for four pairs. Now, should I simply run each one with matching filtering, or build a nice refugium underneath the table and run pvc lines with outlets to each tank. Please let me know what you think. You are the man! Thanks so much. Cal

  • Calvin, Mark asked me to weigh in on this one – in short, there is no right or wrong answer to your question. Clownfish broodstock for the most part have limited space needs (Eg. most Percula and Ocellaris pairs will be fine in a 10 gallon space). Obviously do your research so you can plan for larger species if that’s your anticipation (eg. 20 gallon highs take up not much more physical space than 10s). The upside to running each tank individually as a standalone entity is that a problem on one will not effect the other 3 – particularly helpful as I’ve known more than one clownfish breeder who’s wiped out his broodstock by adding something new to a system without quarantining it first. The downside is that you’ll be slightly increasing your maintenance burden (cleaning 4 filters instead of one). In this particular instance, central filtration will give you the benefit of a larger, shared volume of water, and will allow you to employ more, better equipment (eg. you might not put 4 protein skimmers on 4 small tanks, but buying one skimmer to run all 4…that makes more sense).

  • SEBEAST says:

    Mark! URGENT!
    My nitrates are at 60 ppm and were at 160! Last
    Week. 100 gallon tank.I had a coral cat shark about 14″
    A bamboo shark about 3″. Lionfish about 6″
    And two triggers. They all died two weeks ago after I bought a
    Yello tang, it died of ick or I thought it did and then my fish started getting
    Cloudy eyes, and had spots were their color was gone. They all died.
    I have about 70-80lbs of live rock, sand 40
    Gallon sump, with protein skimmer. I have no fish and just coral I have done 2, 15 gallons water changes but my nitrates are still high.
    I also had a lot of algae and bubble algae. I rearranged my rock so that there would be better flow. Please I need help I have a pair of snowflake clowns waiting for their home to be ready. No ammonia no nitrites but nitrates are high 60ppm. What do I do?
    Thank you

  • Sebeast..I suspect your fish died of marine velvet, not the nitrates. That being said, I don’t recommend you put sharks (of any kind or size) in that tank. They need LOTS of swimming room. Please don’t purchase those animals again.

    Also, 1 lionfish and 1 trigger would be all the fish I’d put in that tank. You could certainly put in some smaller fish assuming the lionfish doesn’t eat them.

    As far as your nitrates, large water changes (over 50%) will bring them down.

    Finally, you’ll need to make sure you leave your tank empty (no fish) for at least 60 days to rob the marine velvet of a host. The full story on how to beat marine velvet can be found in my Disease, Treatments and Quarantine guide.

  • Tom De Meulder says:

    I have a percula 90 all in one. In the back compartiments there is a lightbrown dust piling up. Could this cause nitrates and do i have to suck it out or can i leave it be? Actually what is it?


  • Tom..the light brown dust is detritus. Suck it out next time you do a water change to help keep your nitrates in check

  • Mitun says:

    Hi Mark,

    I have had my tank for a good three years with all the same fish. I have a 55 g tank with one clown, 3 tangs and a blue surgeonfish. All the fish are healthy and eating well. I did a test today and my nitrates are really high but nitrites are undetectable, ph and carbon hardness are also good. Is this normal, am i feeding too much? I also do regular water changes every 3 weeks. Please help.

  • Mitun…your tank is overstocked. I’d whittle your fish down to 1 tang (assuming its a small bodied tank like a Kole tang), the clownfish and then add smaller fish. The other fish need more room especially considering there are multiple of them in that sized tank

  • Mitun says:

    Ok, thanks. So I will upgrade to a bigger tank. What size do you recommend for those 5 fish? Do you think this will help with the Nitrates as I was concerned about that in my previous question.

    Really appreciate your quick response.

  • More water volume will certainly help with nitrates. What tangs specifically do you have?

  • Mitun says:

    I have 2 yellow tangs, 1 kole tang, 1 blue surgeon fish, and 1 black and white clown.

  • Mitun, the surgeon fish will get quite large so I’d recommend at least 300 gallons (1135 L) for him. If you can find him a good home, a 180 gallon tank would be fine to keep the other tangs

  • Gregg says:

    I set up a new saltwater aquarium approx 10 days ago, i intorduced 5 damsels and have been doing two feedings per day, flake in am, grated shrimp in pm. Ammonia is starting to come up, nitrites are flat but Nitrates are way up?, I know there is no way I have cycled however the NITRATE spike has me worried, 40 gallon, crushed coral subsrate, eheim 2213, stacked with ceramic beads, very coarse filter, coarse filter, carbon pad and fluff and 36″ fine aeration wand. should i be concerned and do a water change 20% to lower nitrate or ride it out?

    Thanks in advance

  • Michael says:

    Mark I recently tested my 75g tank and my nitrates were 80ppm and my KH was 17. I don’t have a skimmer yet and I have a HOB marineland 400 bio-wheel filter. I also have 1 clown,2 damsels, 1 swallowtail angel and 1 cardinal and 100lbs of live rock about 1-2 inch bed. How do I get my nitrates and KH down? Had the tank about 5 months. Also have the power head at the top back corner facing toward the middle of the tank. Please help. Love my new hobby!

  • Damion says:

    Well, I guess I have a Silver Bullet…
    I’ve been using the Natureef Denitrification system for 8 years. My Nitrates are always 0. I have a 90Gal. reef tank with a heavy bio load of 16 fish. I feed 3 to 4 times a days. Besides my tank upgrade, I have done 3 water changes in 8 years, with parameters very close to natural seawater all along.
    The Natureef system is a little tricky to get started, but once it’s going, a little maintenance is all that’s needed. Best Tank investment Ever!

  • Meelan says:

    Hi Mark

    I have a 50L tank which has been running for 5 months now. I recently started having allege problem even though I’m doing water changes. I have a test kit called Tetra Marine test 5in1. The problem I’m having is that all my levels according to the test are in range but their is till allege growing in my tank. I have green and red allege and they still comeback after doing water changes every month. Is their anything that I can do or is just a natural occurrence? Thanks!

  • Jonathan Harmantas says:

    When running Bio-Pellets, I’ve found that running the effluent into a Filter Sock captures an incredible amount of sloth that comes off of the bio-pellets as they are consumed. The filter sock is wrapped tightly around the return pipe from the bio-pellet reactor using a zip tie, and I change it every 4 days. You WOULD NOT BELIEVE how much bio-pellet gunk (technical term) gets trapped and removed by this sock. Without the sock my Protein Skimmer gets so dirty that it would almost need to be cleaned every single day. I see no reason to place the burden on the protein skimmer. I am surprised that using a sock isn’t more commonly recommended in conjunction with a bio-pellet reactor.

  • Braden says:

    I am having a HUGE problem with nitrates at the moment!!! I bought a 30l nano as i thought they were cute, and thought i could keep the parameters in check despite the extra maintenance in the smaller tank. However, GCSE`s came along and water changes kinda went out the window for a while!! my nitrates are at 160 ppm and my pH at 7.8. I have done a couple of water changes so far in the past couple of weeks, plus added one dose of tetra nitrate minus and a few doses now of `pH 8.3 powder`. SO FAR NO GOOD!! 🙁 I have the extra problem that i first tested my water at our fish shop, who said it was fine, so i bought a coral! then i realised the shops results were bad, so i am worried for my new coral!

    What shall i do for my nitrates, and will my corals be ok until they come back to normal?

  • Manfred says:

    Hi Mark,
    I would appreciate your advices. I’ve been keeping a saltwater fish only tank (210g) for just over 6 months and have been slowly and gradually adding juvenile size angels and tangs to my tank. Up till 2 weeks ago, I had a mono angel, french angel, rock beauty angel, koran angel, emperor angel, hippo tang and a kole tang. I’ve been changing 20% water every 2 weeks and everything seemed to be going fine till 2 weeks ago when after a water change, the Kole Tang started to look bad and it died the next day. Then, over the course of the next couple of days, I lost another two angels. So I tested my water right away and to my surprise, i found my nitrite level was up and showing in orange color (instead of yellow color that I had showing for almost 6 months). The nitrate level is also up to 10-15 ppm. I did a 20% water change last night but after the water change, I still found my nitrite level high. Do you know why this would happen and any recommendations what I should do? Many thanks!

  • daniel says:

    hi. i have 55 gallon tank. my nitrate is high 40ppm and not going low even i make large water change. but i run bio ring in my sump over 4 year and never change or clean .
    can it be the problem of high nitrate . what i should do ? remove it or change with new one. i worried if i make touch on bio ring filter will changing the bio filter.

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