Nitrate And How To Reduce It

All salt water aquarists will know about nitrate, or at least I hope so. Nitrate (NO3) is a product of the nitrogen cycle, and follows on from Ammonia/Ammonium (NH3/NH4) and [tag-ice]Nitrite[/tag-ice] (NO2). The full nitrogen cycle will lead to nitrogen gas which is removed by gas exchange at air/water interfaces.

Salt water livestock are generally affected by different levels of nitrate. I say generally because both fish and corals, also shrimps etc, have varying degrees of tolerance.

In a fish only aquarium it has been said that [tag-tec]nitrate[/tag-tec] can be allowed to rise to quite high levels, 50 ppm (parts per million) or more. The reason given is that fish can tolerate nitrate within reason. I read of one aquarium where nitrate had risen to nearly 300 ppm. Now that to me is too high! It is true that generally fish can tolerate nitrate, but there are three points to be made on this. Firstly, some fish tolerate nitrate better than others, so an increasing nitrate level could cause trouble. Second, it has been suggested, and it seems reasonable, that if the nitrate level increases slowly, then the fish will ‘learn’ to tolerate it, that is, they will physically become accustomed to it. This seems to be supported by some anecdotal reports of new fish being introduced to fish only aquariums with a known high nitrate level, only for the fish to run into serious difficulties and have to be removed. There wasn’t any disease in the aquariums and the resident fish were fine. Third, why should fish be exposed to high nitrate levels anyway? They don’t have this exposure on the reef. They shouldn’t be exposed to it in the aquarium. This, to me, is simply being a caring aquarist.

The reef aquarium containing corals and probably fish should not be exposed to high nitrate levels. There has never been, to my knowledge anyway, any suggestion by anyone that nitrate can be permitted to rise in a reef aquarium. Reef aquarists tend to battle nitrate levels to keep it constantly at a low level. This is just as it should be.

Right, so nitrate should be at low levels. What is a low level? In the reef aquarium the measured nitrate should be 10 ppm or less. I would suggest that in the fish only aquarium the level should be the same. I accept that in a fish only system it may be more difficult, as often there isn’t any ‘live’ rock, or perhaps not enough to have a significant effect on the nitrate level, and fish stocks may be high. The aquarist should keep the nitrate as low as possible, and take adequate action to keep it that way.

In a fish only or reef aquarium, do a nitrate check weekly, unless experienced and it is absolutely known that nitrate is not a problem. Feed carefully. Assuming a protein content of 50%, one gram of flake food (approximately a teaspoon full) can convert to 336 ppm nitrate*. Do regular water changes, making sure there isn’t nitrate present in the new water. Use R/O (reverse osmosis) water for the mix.

Another incentive to keep levels in check is that nitrate is a known cause of undesirable algae.

(* Source Baensch Marine Atlas Vol 1)

Reducing Nitrate

The fish only or reef aquarist may state that despite care they have an increasing nitrate level. What should they do to control it?

It is assumed that a water change has been done, or better, water changes are routinely done, but this has not had much of an impact on the nitrate level. Well, this is understandable. Water changes have less impact on conditions than most suppose (however, for several reasons it is recommended that they are done). To try to have an impact on the nitrate, prepare a 50% water change. This should have a noticeable effect. Here is an interesting link about water changes and their effect:

Having done the large water change, the aquarist now has to ensure that the nitrate levels are controlled.

The very first action is to consider feeding, is it being overdone? Is all water quality support equipment working properly and adequately sized (protein skimmers, circulation pumps, general filters etc)?

A. In the fish only tank, the first option is larger routine water changes. This increases expense, of course, and it may not be practical because of the higher gallonage requirement. If this option is chosen, it follows that the aquarist must do regular nitrate tests, at least weekly, to see if the changes are effective. If not, then even larger water changes may be required. At this point, it may be better to consider other means to control nitrate.

Though expensive, the aquarist can consider obtaining ‘live’ rock. This must be in sufficient quantity to have an adequate filtration effect. Too little, and though there will be some benefit, the nitrate will not be adequately reduced. This option is not usually chosen by a fish only aquarist with a heavily stocked system. Likewise, if a sump is not already present, this option is not usually chosen (a sump can have a DSB (deep sand bed) installed which has a filtration effect). Anyway, probably the sump option would also not adequately deal with the waste produced by a heavily stocked fish only system.

The fish only aquarist will probably be better off with a de-nitrification filter, possibly in combination with other anti-nitrate measures. These filters are available commercially. There are two main types, one needs feeding special foods for the bacteria and the other does not. The filters rely on bacteria, and these bacteria must have an environment with very low oxygen. The manufacturer’s instructions must be followed carefully. Both, it has to be said, can be a little fiddly to operate. The reason for this is that the flow rate is very slow indeed during maturation, and not much quicker after – at this point the flow rate is around a very fast drip. However, they are effective, and are designed for continuous use. It is also a fairly simple task for DIY. Whether DIY or purchased from a commercial source, the type that contains sulphur beads is recommended. This is because the sulphur type is a little more forgiving and there isn’t a feeding requirement. A check each day to ensure that the flow rate is correct is all that is normally required. I have used a sulphur unit for years. For information on sulphur de-nitrification follow this link:

If the de-nitrification by filter option is taken, the aquarist can supplement its action by installing a DSB and algae (Caulerpa) in the sump (if there is one). This algae can also be installed in the main aquarium, but in the presence of tangs may become a welcome meal. If the algae is used then it must be harvested from time to time, always leaving enough to permit adequate re-growth.

B. If the aquarist has a fish only tank that already has ‘live’ rock, then a check of the quantity of rock should be performed. If the quantity is inadequate relative to the net gallonage, purchase some more. If this is not attractive or does not do the trick, particularly in the presence of a heavy fish load, then the options at (a) will apply.

C. The odds are that reef aquariums will already have ‘live’ rock. Again, a check should be made that there is sufficient rock present, that it is reasonably clean, and that water circulation is adequate. If there isn’t a sump present, then it is recommended that one is added and either a DSB or Plenum (a raised DSB) is installed. This will generally enhance the environment as well as introduce additional filtration. The introduction of algae (Caulerpa) is again a further method of reducing nitrate. There is no reason why filters for de-nitrification cannot be considered, but in a reef aquarium I would suggest, after a check of the quantity of rock, that it is best to consider the sump and DSB/Plenum first.

If the water used for exchanges is prepared free of nitrate, and the aquarist is sure his/her maintenance methods are correct (such as feeding), using some or even all of the suggested remedies should prove effective.

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  1. Where can I purchase sulfur chunks to build a denitrator?

  2. Hello Tim.
    Sulphur is available from two sources.
    First, try a drugstore (also called pharmacy, or chemist). If this is a no go, the second option is to obtain sulphur that is available as a replacement for a commercial sulphur denitrator. Online stores such as may be able to help.
    If not, try an approach direct to the manufacturers.
    The second option will supply sulphur that is dimensionally exactly right for a denitrator.

  3. Hi,
    I have had high Nitrate levels 40-60ppm for a couple of weeks.
    I have increased live rock to total 20kgs for a 250lit tank
    Is this enough? Also I was told to increase circulation by intoducing a Tunze Nanostream. Is this necessary and if so does it have to run 24hrs?

  4. Hi.

    The usual guideline for the amount of live rock that is required is given as 1.5 to 2lbs per gallon. This guideline is however flawed by the fact that differing live rock types have differing weights. Therefore, once the type of rock is known, a query could be made to the LFS on the amount needed. Another way that I saw mentioned on the internet is that if the system is a reef then about two thirds or so of the volume should be rock (ie the reef itself) and provided there isn’t any overstocking then all should be well. I haven’t tried this method.

    Circulation is indeed important for the health of the aquarium – the bacteria cannot operate efficiently if they do not receive the toxins to process. Nitrate won’t be processed unless the bacteria can get at it (different bacteria to those processing ammonia and nitrite). Circulation should be on for 24 hours (except perhaps at feeding time when some apparatus permits the flow to be stepped down temporarily).

    The wording ‘within reason’ is used with live rock and its ability to deal with nitrate. If the feeding regime is poor, that is, the food input to the system is excessive, then the nitrate could rise faster than the bacteria can deal with it.
    .-= John´s last blog ..Ouch! Know Your Enemy =-.

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