Nutrient control: nitrate and phosphate in a reef tank

The nitrate and the phosphate are the two nutrients that the reef keeper manages more than any others, and the nitrate and the phosphate are the two nutrients that, when mismanaged, produce the most dramatic tank problems. The reef with the right nitrate and the right phosphate is the reef with the healthy coral growth, the clean glass, the minimal algae, and the vibrant coloration. The reef with the wrong nitrate and the wrong phosphate is the reef with the algae overgrowth, the coral bleaching, the cyanobacteria, and the dinos. The nutrients are the foundation of the reef chemistry, and the nutrient management is the foundation of the reef husbandry.

This article is going to walk you through the practical management of nitrate and phosphate in a reef tank — the target values for the different coral types (SPS, LPS, soft corals, mixed reef), the export methods that work (the protein skimmer, the macroalgae refugium, the water changes, the phosphate-removing media, the carbon dosing), the common testing mistakes, the troubleshooting framework for the most common problems, and the four mistakes that turn a stable reef into a nutrient disaster. I am a reef hobbyist with 15+ years of experience ranging from nano-reefs to 300-gallon mixed reef systems, and the nutrient management is the variable I get asked about most often. The cases that go well share a common feature: the owner has tested the nutrients regularly, has matched the export to the bioload, and has adjusted the management based on the trends. The cases that go poorly share a different common feature: the owner has not tested, has not matched the export, and has not adjusted based on the trends.

I want to be clear at the start: the nutrient management is not a one-time setup. The nutrient management is an ongoing process, and the ongoing process is the difference between the stable reef and the unstable reef. The owner who sets up the export and assumes the export is sufficient is the owner who is surprised when the nutrients drift. The owner who tests and adjusts is the owner whose reef is stable.

The target values: SPS vs LPS vs mixed

The right target for the nitrate and the phosphate depends on the coral type that the reef is dominated by, and the right target is the foundation of the management.

SPS-dominant reef. The SPS corals (the Acropora, the Montipora, the Pocillopora, the Seriatopora) prefer the lowest nutrient levels, and the SPS corals are the most sensitive to the elevated nutrients. The SPS coral's color and growth are at their best when the nutrients are in the "low but not zero" range:

• Nitrate: 0.5–5 ppm. The SPS tank that is consistently above 10 ppm is the SPS tank that is producing the zooxanthellae overgrowth (the brown jelly), the algae overgrowth, and the coral stress. The SPS tank that is consistently at 0 ppm is the SPS tank that is producing the pale, washed-out coloration (the "starving" look) and the increased susceptibility to the disease.

• Phosphate: 0.01–0.03 ppm. The SPS tank that is consistently above 0.05 ppm is the SPS tank that is producing the algae overgrowth and the coral stress. The SPS tank that is consistently at 0 ppm is the SPS tank that is producing the pale coloration and the increased susceptibility to the disease.

LPS-dominant reef. The LPS corals (the Euphyllia, the Catalaphyllia, the Trachyphyllia, the Acanthastrea) tolerate higher nutrient levels than the SPS corals, and the LPS corals often show enhanced coloration and growth at slightly elevated nutrients:

• Nitrate: 5–10 ppm. The LPS tank that is consistently above 20 ppm is the LPS tank that is producing the algae overgrowth and the coral stress. The LPS tank that is consistently at 0 ppm is the LPS tank that is producing the pale coloration and the increased susceptibility to the disease.

• Phosphate: 0.03–0.05 ppm. The LPS tank that is consistently above 0.10 ppm is the LPS tank that is producing the algae overgrowth and the coral stress. The LPS tank that is consistently at 0 ppm is the LPS tank that is producing the pale coloration.

Soft coral-dominant reef. The soft corals (the Sinularia, the Sarcophyton, the Xenia, the Anthelia) tolerate the highest nutrient levels of the common reef corals, and the soft corals often grow rapidly at the elevated nutrients:

• Nitrate: 10–20 ppm. The soft coral tank that is consistently above 30 ppm is the soft coral tank that is producing the algae overgrowth. The soft coral tank that is consistently at 0 ppm is acceptable for the soft corals, but the soft corals often show enhanced growth at the slightly elevated nutrients.

• Phosphate: 0.05–0.10 ppm. The soft coral tank that is consistently above 0.20 ppm is the soft coral tank that is producing the algae overgrowth.

Mixed reef. The mixed reef (the most common hobbyist setup, with a mix of SPS, LPS, and soft corals) is managed at the SPS-LPS compromise:

• Nitrate: 2–10 ppm. The mixed reef that is at 5 ppm is a mixed reef that is meeting the needs of the SPS and the LPS without being too high for the SPS or too low for the LPS.

• Phosphate: 0.02–0.05 ppm. The mixed reef that is at 0.03 ppm is a mixed reef that is meeting the needs of the SPS and the LPS.

The owner who is managing the mixed reef should target the lower end of the mixed-reef range, with the understanding that the SPS corals in the mixed reef will be the corals that are most affected by the elevated nutrients.

The export methods: what works, what doesn't

The export methods for the nitrate and the phosphate are the tools the owner uses to maintain the target values. The methods vary in effectiveness, and the right combination of methods depends on the bioload, the feeding, and the target values.

The protein skimmer. The most effective export method for the organic waste. The protein skimmer removes the dissolved organic compounds (the proteins, the amino acids, the carbohydrates) before the compounds can be broken down by the bacteria into the nitrate and the phosphate. The skimmer is the foundation of the nutrient export in a reef tank, and the skimmer is the first piece of equipment that should be optimized. The skimmer should be:

• Sized appropriately. The skimmer should process the entire tank volume 3–6 times per hour (for the medium- to high-bioload reef), and the skimmer should be able to produce a dark, dry skimmate.

• Adjusted for the nutrient level. The skimmer can be adjusted (the water level, the air flow, the venturi) to produce more or less skimmate, and the skimmer should be adjusted to match the bioload and the target nutrient level.

• Cleaned regularly. The skimmer cup should be emptied every 1–3 days, and the skimmer pump should be cleaned every 1–3 months.

The macroalgae refugium. The most effective biological export for the nitrate and the phosphate. The macroalgae (Chaetomorpha, Caulerpa, Gracilaria) consumes the nitrate and the phosphate as nutrients, and the macroalgae is the most efficient biological filter for the reef tank. The macroalgae refugium should be:

• Substantial in size. The refugium should be at least 10–20% of the display tank volume, and the macroalgae should fill at least 50% of the refugium.

• Lit on a reverse photoperiod. The refugium should be lit at night (when the display tank lights are off), so the pH swing between the day and the night is reduced.

• Harvested regularly. The macroalgae should be harvested every 1–2 weeks (the harvested macroalgae is the nutrient export), and the new growth should be left to continue the nutrient consumption.

The water changes. The simplest and most reliable export method. The water change removes the nitrate and the phosphate (and the other accumulated waste products) directly, and the water change is the foundation of the long-term nutrient management. The water change should be:

• Regular. The water change should be 10–20% per week for the low-nutrient SPS tank, 20–30% per week for the mixed reef, and 20–40% per week for the high-nutrient LPS or soft coral tank.

• Made with the right water. The water should be made with a high-quality salt mix and RODI water, and the water should be tested for the alkalinity, the calcium, and the magnesium before being added to the tank.

The phosphate-removing media. The most effective targeted export for the phosphate. The phosphate-removing media (GFO, lanthanum-based media, the iron-based media like PhosBan) is placed in a media reactor in the filtration system, and the media binds the phosphate and removes it from the water. The media should be:

• Used in the right amount. The media is dosed based on the phosphate level, and the dosing is adjusted based on the testing.

• Replaced regularly. The media has a finite capacity, and the media is replaced every 1–3 months (the exact timing depends on the phosphate level and the tank size).

The carbon dosing (Vodka, vinegar, biopellets). The most effective biological export for the nitrate (in some cases). The carbon dosing provides a carbon source for the heterotrophic bacteria, and the bacteria consume the nitrate as they process the carbon. The carbon dosing is a powerful tool, but the carbon dosing can produce the bacterial blooms, the dinos, and the cyano blooms if the dose is too high or if the dosing is not matched to the tank. The carbon dosing should be:

• Started slowly. The dose should start at the low end of the recommended range, and the dose should be increased gradually over 2–4 weeks.

• Monitored carefully. The nitrate, the phosphate, and the bacterial population should be monitored, and the dose should be adjusted based on the trends.

• Used with the right tank. The carbon dosing is appropriate for the high-bioload tank with the elevated nutrients, but the carbon dosing is not appropriate for the low-bioload tank with the already-low nutrients (the carbon dosing would crash the nutrients to zero).

The common testing mistakes

The testing is the foundation of the management, and the testing errors are the most common source of the management errors.

The expired test kit. The test kit that is past the expiration date is the test kit that is producing inaccurate results, and the inaccurate results are producing the wrong management decisions. The fix is to check the expiration date on the test kit, and to replace the expired kits.

The uncalibrated test kit. The test kit that has not been calibrated (the reference sample is not testing as expected) is the test kit that is producing inaccurate results. The fix is to use the reference sample to verify the kit's accuracy, and to replace the kit if the reference sample is not testing correctly.

The wrong test for the parameter. The nitrate test that is designed for freshwater is the test that is producing inaccurate results in the saltwater, and the phosphate test that is reading the total phosphate (rather than the orthophosphate) is the test that is overestimating the bioavailable phosphate. The fix is to use the saltwater-specific test kits (the Salifert, the Red Sea, the Hanna) for the saltwater tank.

The testing at the wrong time of day. The nitrate and the phosphate fluctuate over the course of the day (the nutrients are produced by the fish and the feeding during the day, and the nutrients are consumed by the algae, the bacteria, and the corals at night). The testing should be done at the same time of day (typically in the morning, before the lights come on, when the nutrient level is at its most consistent). The fix is to establish a testing routine, and to test at the same time of day, every time.

The testing without the proper water sample. The water sample that is taken from the surface (where the nutrients are typically higher) is the sample that is overestimating the tank's average nutrient level. The water sample that is taken from the substrate (where the nutrients are typically higher) is the sample that is overestimating the tank's average nutrient level. The water sample should be taken from a representative location (typically from the mid-water column, away from the substrate, away from the surface, and away from the filter outlets). The fix is to establish a consistent sampling location.

The troubleshooting framework

The troubleshooting framework for the most common nutrient problems:

Problem: Nitrate is consistently above the target. The possible causes and the matching fixes:

• Overfeeding. Reduce the feeding, and remove the uneaten food promptly.

• Insufficient protein skimming. Optimize the skimmer (the water level, the air flow, the venturi), and clean the skimmer cup and pump.

• Insufficient water changes. Increase the frequency or the volume of the water changes.

• Insufficient macroalgae in the refugium. Add more macroalgae, and harvest the macroalgae more frequently.

• High bioload. Reduce the bioload (rehome some fish, return some corals), or upgrade the export.

Problem: Phosphate is consistently above the target. The possible causes and the matching fixes:

• Overfeeding. Reduce the feeding.

• Insufficient protein skimming. Optimize the skimmer.

• Insufficient water changes. Increase the water changes.

• Insufficient macroalgae. Add more macroalgae.

• Source water is high in phosphate. Test the source water, and switch to RODI water if the source water is the problem.

• Insufficient phosphate-removing media. Add a media reactor with the appropriate media.

Problem: Nitrate or phosphate is consistently at zero. The possible causes and the matching fixes:

• Excessive export. Reduce the export (reduce the water changes, reduce the macroalgae harvest, reduce the carbon dosing).

• Insufficient bioload. The bioload is too low to produce the measurable nutrients, which is a sign of a very low-nutrient tank (the SPS tank, with very few fish and very heavy export). The fix is to add some nutrients (the targeted feeding of the fish foods, the use of a balanced nutrient supplement) to bring the nutrients into the "low but not zero" range.

• Test inaccuracy. Verify the test with a reference sample, and recalibrate or replace the test kit.

The four mistakes that turn a stable reef into a nutrient disaster

Mistake 1: Not testing the nutrients regularly. The most common mistake. The owner does not test the nitrate or the phosphate for months at a time, and the nutrients drift (up or down) without the owner knowing. The fix is the weekly testing, with a consistent routine, with the right test kits, and with the trends recorded in a log.

Mistake 2: Crashing the nutrients to zero. The second most common mistake. The owner uses the heavy export (the carbon dosing, the media reactor, the frequent water changes) to drop the nutrients quickly, and the owner crashes the nutrients to zero. The zero-nutrient tank is the tank with the pale coral, the increased disease susceptibility, and the long-term decline. The fix is to reduce the export, and to bring the nutrients into the "low but not zero" range.

Mistake 3: Using the wrong salt mix or the wrong source water. The third mistake. The salt mix that is high in nitrate or phosphate is the salt mix that is adding to the nutrient load with every water change. The source water that is high in nitrate or phosphate is the source water that is contributing to the nutrient load. The fix is to use a high-quality salt mix (the Tropic Marin, the Red Sea Coral Pro, the Aquaforest) and the RODI water.

Mistake 4: Overcorrecting the nutrient levels. The fourth mistake. The owner sees a high nutrient level, the owner makes a major export change, the owner overcorrects, the nutrients crash to zero, the owner makes a major export change in the other direction, the nutrients spike, the cycle continues. The fix is to make small, gradual changes to the export, and to allow 1–2 weeks between changes for the system to respond.

The bottom line

The nutrient management is the foundation of the reef husbandry, and the nutrient management is the variable that the owner can adjust most easily. The right target is the species-appropriate target (the SPS-dominant reef, the LPS-dominant reef, the mixed reef). The right export is the combination of the protein skimmer, the macroalgae refugium, the water changes, the phosphate-removing media, and (in some cases) the carbon dosing. The right testing is the weekly testing, with the right kits, at the right time, at the right location, and with the trends recorded in a log.

The owner who is testing and adjusting is the owner who has a stable reef. The owner who is not testing and adjusting is the owner who has a reef that is drifting. The difference is the management. The management is the framework above. The framework, applied with consistency, is the right prevention.

The reef that is well-managed is a reef that is beautiful, that is stable, and that is a source of joy for the owner. The reef that is poorly managed is a reef that is a source of frustration. The difference is the management. The management is the framework above. The framework, applied with consistency, is the right prevention.

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About the author

Olivia Brown is a long-time reef hobbyist and consultant with 15+ years of experience ranging from nano-reefs to 300-gallon mixed reef systems. She has worked with both hobbyists and small commercial aquaculture operations on tank cycling, husbandry, equipment selection, and the long-term management of the reef ecosystem, with a particular focus on the practical realities of maintaining the stable parameters that the reef corals need to thrive. She writes regularly on the day-to-day realities of keeping a reef tank healthy and the coral vibrant.