Oh, no, nitrate…

…and I don’t know why. I’ve live rock and everything seems fine at the moment, but I’m worried..

These words are quite understandable. Over and over again mention is made of seawater quality and how important it is – in fact seawater quality is the number one requirement, ahead of lighting (for a reef aquarium). So concerns in this direction are quite correct.

First, without being over scientific, what is nitrate? Every aquarium in order to be healthy needs a biological support system, commonly called a bio-filter, though some aquarists call it aquarium life support. This filter when functional is loaded with bacteria. The life processes of livestock and the rotting of uneaten food, algae and the like creates toxins, the first of which is ammonia. This is deadly to livestock. Bacteria within the bio-filter convert the ammonia to nitrite, again nitrite is a toxin and nearly as deadly as ammonia. Other bacteria then convert the nitrite to nitrate which in general is not toxic, but detrimental to seawater quality at too high a level. The bacteriological process has the overall title of ‘The Nitrogen Cycle.’ This cycle under certain conditions continues when nitrate is converted to gas which escapes from the aquarium, but here the concern is nitrate.

In the aquarium, whatever type it is, ammonia, nitrite and nitrate are being continually produced. The production is at its highest in a fish only system, as fish consume relatively large amounts of food which enters the Nitrogen Cycle process. Reef systems have lower fish loads, but nevertheless the same applies. The bacteria are continually hard at work.

The guideline that is given for nitrate when tests are being done is 10ppm (parts per million) or less in a reef system, and as close as possible in a fish only system. Why ‘as close as possible?’ It is more difficult to maintain a low nitrate presence in a fish only system because, as previously stated, there is a higher fish load and consequently heavier feeding. Why is the level stated more strictly for a reef system? This is because there are corals present and these are generally more sensitive to nitrate than fish. Note that some fish are more sensitive which should be noted in research before purchase.

Just to wander off the point a moment, there is something else that is interesting as well. I have never encountered this problem. A good while ago a well-known aquarist was asked in a magazine why a recently purchased fish, stated by authorities to be ‘reasonably hardy’, on introduction to a fish only system died. All the other fish present, some considered to be less hardy than the new addition, were fine. After a considerable amount of head scratching, it transpired that the nitrate reading in the seawater was approaching 150ppm. A little high in anyone’s book! There wasn’t any mention of problem algae which was fortunate for the aquarist. The fish that were already in the aquarium had been there from the word go or a little later. It was also discovered that seawater changes were done very spasmodically. So why did the new fish die? It had been purchased from a retailer whose seawater quality would have been at least reasonably good with low nitrate. The fish had then been introduced to very polluted seawater and had succumbed. What of the other fish? These had been in the aquarium for a long while and the nitrate level would have risen slowly. So they had become accustomed, or perhaps it is better to say ‘hardened’, to the nitrate presence. The advice was to carry out regular seawater changes and bring down the nitrate level, but not too quickly in case of any affect on the resident fish.

I bet that problem doesn’t happen very often. Anyway, back to the text.

There’s nitrate present – what to do? If the nitrate is within the guidelines, then perhaps it could be lowered even further by a small increase in the amount of seawater changed at routine changes. It is when it is quite high that is of most concern. The seawater needs to be of high quality and there isn’t any desire to tempt fate by providing nourishment for nuisance algae.

The first action is to attack the problem by, as just mentioned, increasing the amount of seawater changed. The basic general guideline for routine seawater changes is 10% of the net gallonage in the whole system but this can be flexed according to need. It is best not to increase the amount above 25% (severe cases) as the ‘raw’ seawater in this quantity isn’t always happily received by livestock. Increasing the amount changed increases dilution. The level should come down slowly until it is acceptable. The continuous addition of food and presence of livestock means that the Nitrogen Cycle is ever active (as it must be), so routine seawater changes should continue. After the following, if a cause of excessive nitrate is found the amount of the changes could be reduced again.

The above is treating the effect but what about the cause? It is best to start from square one and work through. There isn’t anything difficult in the process.

Excessive nitrate is in the seawater, but could it be getting in before any seawater enters the aquarium? First, have a look at the dry salt mix in use. It would be unusual with modern salts for there to be any nitrate (or phosphate) presence, but check that this is so by looking at the manufacturer’s information. Often the dry salt package will advise the salt is nitrate and phosphate free. In the unlikely event it isn’t free of these pollutants, change the brand to one that is.

The dry salt has obviously to be mixed with fresh water before use. It is highly recommended that RO (reverse osmosis) water is used, which is tap water that has been ‘super filtered’ to remove any unwanted substances. If RO water is in use, do a nitrate test to see if the unit is functioning properly. If there is a nitrate presence, then the heart of the RO unit, the membrane, may have a problem, and this should be checked. It may mean the purchase of a replacement unit.

If tap water is in use and not pre-filtered, then the same thing applies – test the water. In some areas tap water is fairly pure with a very low nitrate content, but in other areas it may not be. These areas are often agricultural where there is a lot of fertilizer in use (including nitrate), or industrial. If it is found that there is a nitrate presence, particularly a fairly high one, then it is best to invest in an RO unit and be rid of the problem. There isn’t much point in introducing nitrate when fresh seawater is mixed after the manufacturer has taken the trouble (which the aquarist pays for) to ensure the salt is nitrate free.

The next checks concern the aquarist’s husbandry disciplines. The aquarist can quite easily be over or under enthusiastic in some areas.

The first area to consider is feeding, where the majority of new aquarists overdo it. Overfeeding is fairly difficult to avoid by an experienced aquarist, as it is hard to ensure that all food goes where it is intended. The new aquarist is often over-anxious that the livestock get enough to eat and consequently too much food gets into the system, much of it is not eaten, it rots and joins the Nitrogen Cycle. This cycle as already stated produces nitrate. As a point of interest, as with nitrate, phosphate is linked to nuisance algae outbreaks. Guess how the majority of phosphate gets into the seawater – yes, with food. So it is very important not to overfeed. There are a few guidelines on feeding techniques to look at, including one under ‘Articles’ on this site titled ‘Food For Thought.’ Allowing a little time to learn how to feed and ensure the fish have enough, and at the same time avoid overfeeding as far as possible, is very worthwhile.

The second discipline that needs to be looked at is maintenance. Some aquarists like doing this (as some gardeners like weeding) and some do not. I don’t completely enjoy it, but do it willingly because the appearance of my reef, meaning the health and vitality of my livestock, depend on it. So how does maintenance have any effect on nitrate?

Good seawater circulation usually means good oxygen content. Oxygen is taken in at air/water interfaces, particularly the seawater surface in the aquarium. If circulation is at it should be oxygen content should be adequate. The bacteria that convert the toxins ammonia and nitrite are very reliant on oxygen availability in the seawater, so if there is plenty always available then the bacteria are able to function efficiently. This produces nitrate as previously mentioned. If live rock is used in the aquarium, and the oxygen loving bacteria are producing nitrate, then the nitrate converting bacteria convert it to gas, which easily escapes from the aquarium because of the good circulation. So the aquarist needs to ensure that circulation is adequate. Lack of good circulation could be a design error in a new aquarium, or in an established one the result of coral growth blocking seawater flow, or powerhead intakes being clogged with detritus, or tubes becoming narrow with detritus build-up. Checking these is part of ongoing maintenance. Intakes can easily be checked visually and the inside of tubes occasionally – seawater output, or the lack of it, could indicate tube blocking.

Detritus itself is a normal entrant to the aquarium. When maintenance is being done detritus should be removed as far as possible. This removal helps prevent rotting substances from entering the Nitrogen Cycle in the first place, thus not producing any additional nitrate. If detritus is heavy an investigation should be made to find out why and hopefully remove the cause. Seawater circulation usually means that detritus tends to settle in one or two particular areas, usually where the flow is lower. When a routine seawater change is done this detritus should be siphoned out.

Detritus can also settle like a dust on live rocks, or any rocks for that matter. It is a good idea to remove it once in a while. How can that be done? Obtain a baster (as used in a kitchen) or similar and gently pump seawater over the rocks. This will put the detritus in the seawater column and hopefully permit either a mechanical filter to extract it, or make it settle in an area where it can be siphoned out.

Another area to check is a sand bed. This sand bed could be in the display aquarium or in a sump, though in this case the sump is unlikely – unlikely because the sand bed here is one for decorative purposes only which would not normally be in a sump. The decorative bed is usually 1 to 2″ deep and made up of coarse coral sand. They can be very attractive. They can also get very dirty, as detritus settles into the sand bed between the grains. This is unwanted and the detritus should be removed by using, for example, a gravel cleaner on a reasonably regular basis – at least when the sand starts to look dirty. A dirty sand bed is not attractive anyway so the aquarist should notice it. (Note that only a decorative sand bed should be cleaned, not a DSB (deep sand bed))

Finally, nitrate can be produced in excess by the livestock, particularly fish. Not directly of course, but because of their life functions and the food that has to be offered. So it is important not to overstock – the more fish the stronger the pressure on seawater quality. More fish, more food and the harder the bacteria work. Normally they’ll do their job well, and normally they’ll work 24/7 on everything suitable and available.

  1. Nice Blog! Although I’m just starting out with a freshwater tank for now, I do eventually plan on getting into Saltwater once I learn what I’m doing so I’m definitely glad I found this site.

    Great job, keep it up.


  2. Hi! Great article. I am setting up my first mangrove tank in the next month. I can’t wait to see how much nitrate it can remove.
    Macro algae are also a good way to reduce nitrate but I did not tried yet.

    Do you have anything to recommand about those 2 way to reduce nitrate?

    Patrice from http://www.aquariumslife.com

  3. Hi Patrice,

    If you’re interested in nitrate reducing macroalgae, a good safe bet is a refugium or a section of your sump for Chaeto, and Halimeda.

  4. Paul : Thanks for the kind comments

    Patrice : One thing to bearn in mind is that nitrate reduction via mangroves is very very slow. The reason for this is that they grow so slow. Macro algae does remove it at a much faster rate however in my opinion it does not look as nice!. How about using them both – you can never have too many nutrient exports. Just remember to look after them so that they can do their jobs.

  5. Great information, could have done with this information when i had a big problem.

    My addition to this, as with everything in this hobby is good diciplin in maintanance and regular water changes.

  6. Yes, you’re absolutely right there.
    Routine seawater changes and other maintenance – essential.

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