For much of the last two decades in the reef aquarium hobby, the rule of thumb for nitrate (NO3) and phosphate (PO4) has been to keep them as low as possible. One often hears that they should be “undetectable,” i.e. measure zero with a hobbyist test kit. Until relatively recently, this has actually been sound advice, since it was so unlikely (especially with older equipment) that either nutrient would be depleted in a closed system to the extent that it would affect the coral. We are now entering a point of development in the hobby where this scenario is not only possible but a fairly common occurance. Better skimmers and more diverse nutrient reducing techniques, combined with a virtually unlimited and readily available range of lighting for aquaria, has finally made it possible to begin talking about very small amounts of these nutrients being the limiting factor in coral growth, color and health. This is an exciting moment in the hobby, since it appears we are nearing the tipping point eventually achieved in all other forms of farming, husbandry and cultivation where the growth and quality of coral properly raised in aquariums exceeds that of coral in the wild.
Why are nutrients important in a reef aquarium
If either nitrate or phosphate become excessively depleted in an aquarium, you eventually reach a point where many forms of coral, algae, bacteria and other microbes (zooplankton, for example) cannot eat enough to survive and reproduce. If the lack of nutrients continues for a prolonged time, coral can show signs of starvation. Often they lose their color; occasionally it results in tissue necrosis. The lack of nutrients can also limit the growth of beneficial bacteria, coralline algae and other helpful microbes that round out the natural ecosystem of a reef aquarium.
How much Nitrate is enough; too much
It is important to put this in perspective. NO3 and PO4 depletion in aquariums only occurs if the filtering out and consumption of these nutrients exceeds their production from the breakdown of food added to the aquarium. The rate at which they are produced usually corresponds to the biological mass of the fish in the aquarium, which can vary greatly, and the amount of food the owner feeds them. The filtering out of the nutrients—before or after the food is broken down into biologically available compounds—obviously varies according to the effectiveness of the filtration components of the system. The rate at which they are consumed by the coral varies according to the amount of coral in the aquarium and by the amount of light they receive, which places a ceiling on the growth rate of the coral. The analogy I like to use is light is the gas pedal for coral growth, and nutrients (along with the basics like alkalinity and calcium) are the fuel. If you rev the engine, you need to make sure you are watching your fuel availability. (Some types of biological filters such as Jaubert plenums and deep sand beds probably also significantly alter the consumption of these nutrients in aquariums, but of course can be considered part of the “filter.”)
Thus, a system is most likely to become nutrient deficient if: few or no fish that require feeding have been added to a system with an extremely efficient filtration and extremely strong lighting. A system is least likely to become nutrient deficient if it is heavily stocked with fish, has less than optimal filtration, or has very little coral and/or utilizes weak lights that produce slow or negligible coral growth. The reason most people up to now have not had to add nitrate or phosphate to their aquariums is because most aquariums tend to have an average number of fish with moderately efficient filtration and moderately strong lights.
What is this Redfield ratio we hear about in the hobby
The Redfield ratio is the observation that phytoplankton anywhere in the ocean, taken in aggregate, consists of a relatively stable atomic ratio of 117 parts carbon, 14 parts nitrogen, and 1 part phosphorus (117:14:1=C:N:P. It was originally 106:16:1, but was recently revised) and that this is also the ratio of these elements found in seawater. It is important to remember when thinking about the Redfield ratio that it refers to the composition of microorganisms in ocean water as a whole, not any individual species, and to the dissolved inorganic nutrients in ocean water. It does not refer to the chemical makeup of coral at all or to the exact nutritional requirements of any specific coral species. It is very possible, especially if a large quantity of a single coral species were to be cultivated in a given system, say, for commercial purposes, that the required ratio of C:N:P would turn out to be significantly different. But it is enough to know that the ratio holds true for the composition of natural seawater to use it as a guide for aquarium husbandry in general; these nutrients tend to be taken up in this proportion. As a side note, there is some evidence that other elements can be added to the ratio in trace amounts, especially iron (Fe), which would fall in the range of .1 to .001 per 1 phosphate.
Unlike bacteria, photosynthetic coral produce most of the carbon needed for growth via photosynthesis. This is the reason some aquarists add carbon sources such as vodka, sugar, vinegar and/or “bio pellets.” Because bacteria are not photosynthetic, and because much more carbon than nitrate or phosphate is required for these compounds to be consumed, carbon becomes the limiting factor in bacteria growth—if your goal is to use bacteria to reduce these nutrients.
Because coral is photosynthetic, the amount of NO3 and PO4 in a reef aquarium must match the amount of carbon being produced by the coral, which is ultimately determined by the strength of the lighting used. In most situations, even systems that test “zero” are producing enough for moderate coral growth, assuming the tank has some fish in it that are being fed on a regular basis. The nitrate and phosphate in the tank is simply being consumed immediately as it becomes available. But in heavily stocked tanks with strong lighting and a light fish load, it is actually quite easy to end up with a nutrient deficiency of some kind, whether moderate or severe.
Balancing Nitrate (NO3) and Phosphate (PO4) in your reef aquarium
There are some fine points that must be understood about balancing nitrate and phosphate in a reef aquarium. The Redfield ratio implies that coral and bacteria consume significantly more NO3 than PO4, approximately fourteen times more.
In addition, most of the methods aquarists use to reduce nutrients remove more nitrate than phosphate. This is why a nitrate deficiency is more common than a phosphate deficiency, although this does not account for the proportion of NO3 and PO4 that is being produced in the aquarium in the first place, which probably varies according to the types of food that are added.
If your system tests low for NO3 and/or PO4 (Reef Nation recommends maintaining NO3 between .5-10 ppm and PO4 between .01-.07 ppm) and your corals show signs of nutrient deficiency, there are two obvious paths you can take: reduce your filtration or increase your feeding. However, before you turn off your skimmer and dismantle your refugium and turf scrubber, you must understand the role they play in the aquarium besides nutrient reduction. Skimmers oxygenate the water, increase gas exchange, reduce CO2 and help stabilize the pH. They also remove much more than NO3 and PO4 from the water, and will help prevent a chain reaction in the event of a sudden fish or coral death. Refugiums grow beneficial microorganisms as well as zooplankton and algae scrubbers remove many toxic metals. All of these components serve a function, and if modifications are made they should probably be modifications of size, strength or flow rate rather than complete removal.
To this end fish may be added to the aquarium, especially larger fish that eat more and live longer. It is also possible to use old water from a fish-only tank for this same purpose, as long as the tank is stable and disease-free and no livestock has recently been added. Many fish-only systems—especially ones with large angels, pufferfish, triggers, or other non-reef-safe carnivorous fish—are maintained at nitrate levels of 40-80 ppm or higher. One must simply test the water to determine its concentration of nutrients and use it to “spike” the new water used for water changes on your reef. This is labor intensive, but has the advantage of being completely natural.
Correcting a common issue naturally: High Phosphate/Low Nitrate
But if nitrate levels are low and phosphate high, the only way to balance the nutrient deficiency is to add nitrate alone. If a system is nitrate deficient, dosing NO3 alone will cause PO4 to fall, similar to the way carbon dosing causes nutrients to fall or adding calcium to a calcium deficient system will cause the alkalinity to fall; often one nutrient cannot be consumed without the presence of another. This is actually the best way to reduce elevated PO4 levels, since it also encourages growth.
Dosing nitrate in your reef aquarium
The most common way to dose NO3 is to use plant fertilizer. Nitrate has been used as a fertilizer at least since the Middle Ages, however it is always bound to another element to produce a salt. The most common forms are potassium nitrate (KNO3) and sodium nitrate (NaNO3). The problem with using only one form of nitrate over a long period of time is that you end up adding the other element as well, which can but is unlikely to cause an imbalance of that element. KNO3 has an N-P-K rating of 13-0-44, which means it contains roughly three times as much potassium as nitrate (and no phosphate). NaNO3 has an N-P-K rating of 16-0-0, with the bulk of the remainder presumably being sodium. Potassium occurs in seawater at roughly the same concentration as calcium (K approximately 390 ppm and Ca 420 ppm), so continual dosing of KNO3 to maintain NO3 levels at .5-2ppm may also increase K levels above natural seawater. NaNO3 may be a better choice for long term use, since it is virtually impossible to overdose sodium in saltwater, as it is the Na in NaCl (salt). Saltwater contains more than eight times the amount of sodium as magnesium.
Keep the big picture of nutrient balance in mind
While many of us in the reef hobby love to tweak our systems through the addition of lights, carbon dosing, bio pellets, refugiums, scrubbers, additives etc, the deeper we dive into how the nutrient balance of our systems work shows us that we need to take a measured approach when doing so. It is easy for us to add equipment and additives to our systems, but if we dive first in to the data to determine what our current state is, what our goal is, and how the system is functioning today, we should be able to dial up the desired outcome much easier and with a lot less cost and effort.