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Nutrients are food, not just pollution
For decades, the reef hobby treated nitrate and phosphate as pure enemies. The goal was "zero" on every test. Today most experienced reefers aim for low but detectable levels. Corals host symbiotic algae (zooxanthellae) that need nitrogen and phosphorus to photosynthesize and share sugars with the coral. Starve the algae and corals pale, polyp extension drops, and growth slows. Overfeed them and nuisance algae thrive while some corals brown out.
The aim is a stable middle ground, matched to the corals you keep.
Common targets
| System | Nitrate (NO3) | Phosphate (PO4) |
|---|---|---|
| Softie / LPS / mixed reef | ~5–20 ppm | ~0.05–0.1 ppm |
| SPS-dominant | ~2–10 ppm | ~0.03–0.08 ppm |
| Ultra-low nutrient system (ULNS) | <1–2 ppm | <0.03 ppm |
| FOWLR | Up to ~40 ppm tolerated by most fish | Mostly an algae concern |
These are widely shared hobby ranges, not hard thresholds. A bigger danger than any single value is a nutrient level that crashes to zero or doubles in a week.
Do ratios matter?
You'll often hear the Redfield ratio, about 106 carbon : 16 nitrogen : 1 phosphorus by atoms in ocean plankton. Converted to the ppm units on test kits, that 16:1 N:P ratio is roughly a 10:1 NO3:PO4 ratio by weight. Some hobbyists use it as a rough guide: if nitrate is 10 ppm, phosphate around 0.1 ppm keeps neither nutrient wildly limiting. It is a useful sanity check rather than a precise target, and many successful tanks run far from it. The practical lesson is that one nutrient bottoming out while the other is plentiful is often the setup for cyanobacteria or dinoflagellates.
Measuring correctly
- Phosphate versus phosphorus. Some popular checkers, such as the Hanna HI736 Phosphorus ULR, report phosphorus (P) in parts per billion. To convert to phosphate (PO4) in ppm, multiply ppb P by 3.07 and divide by 1,000. For example, 30 ppb P ≈ 0.092 ppm PO4. Mixing these units up is one of the most common mistakes in the hobby.
- Low-range kits. Standard hobby nitrate kits are hard to read below about 1–2 ppm. If you run low nutrients, use a low-range kit or a digital checker.
- Same time, same method. Phosphate in particular can vary through the day with feeding. Test at a consistent time.
- Verify with ICP. An ICP test reports total phosphorus, which can read higher than colorimetric phosphate kits because it includes organic phosphorus.
Where nutrients come from
Nearly all nitrate and phosphate enter through food. Fish and corals use some for growth and the rest becomes waste. Additional sources include tap water used instead of RO/DI, some dry rocks that leach phosphate, decaying detritus in sand and sumps, and some coral foods and amino acid additives. Phosphate stored in rock and sand can leach back out for months after you lower it in the water column.
How to lower nitrate and phosphate
Feed thoughtfully
The cheapest nutrient export is not importing as much. Feed what fish eat within a couple of minutes, rinse frozen food, and avoid broadcast coral foods if you don't need them. Never starve fish to fix a nutrient number. Use export instead.
Water changes
A 10% water change removes about 10% of dissolved nitrate and phosphate. Water changes are reliable and also replenish trace elements, but they're expensive per ppm removed in a high-nutrient tank.
Protein skimming
A skimmer removes dissolved organic compounds before they decompose into nitrate and phosphate. It does not remove nitrate or phosphate directly, but it lowers the load that becomes them.
Refugium and macroalgae
Growing chaeto (Chaetomorpha) or other macroalgae in a lit refugium takes up both nutrients, and you physically remove them by harvesting. Refugiums also house copepods. They're gentle and self-limiting, which makes them hard to overdo. An algae turf scrubber works on the same principle.
Phosphate media: GFO and lanthanum
Granular ferric oxide binds phosphate. Run it in a reactor and start with a smaller amount than the manufacturer's maximum, because a sudden drop in phosphate can shock SPS. Aluminum-based media also work, though aluminum leaching has been blamed for coral irritation. Lanthanum chloride precipitates phosphate quickly and cheaply, but it must be dosed slowly and run with a fine filter sock or filter, because lanthanum carbonate and phosphate particulates can harm fish.
Carbon dosing
Dosing an organic carbon source such as vodka (ethanol), vinegar (acetic acid), or commercial products like Red Sea NO3:PO4-X or Brightwell's offerings feeds heterotrophic bacteria. Those bacteria grow, take up nitrate and phosphate in the process, and are removed by the skimmer. Carbon dosing can drop nitrate dramatically, but start at a tiny dose and increase slowly over weeks. Overdosing can deplete oxygen or produce a bacterial bloom, and carbon dosing needs a good skimmer. It also tends to deplete nitrate faster than phosphate, which can create the zero-nitrate conditions that dinos like.
Denitrification
Low-oxygen zones in deep sand beds (DSB), dense rock and special denitrator reactors host bacteria that convert nitrate to nitrogen gas. These work passively but are slower to adjust.
How to raise nutrients that are too low
A tank reading zero on both nitrate and phosphate with pale corals or dino/cyano problems usually needs more nutrients, not fewer:
- Feed fish more, or add fish.
- Reduce export: smaller skimmer cup, less GFO, less carbon dosing, smaller refugium light period.
- Dose directly. Sodium or potassium nitrate raises nitrate, and sodium or potassium phosphate (or products like Brightwell NeoNitro and NeoPhos) raises phosphate. Aim for small daily increases and retest.
Ultra-low nutrient systems (ULNS)
A ULNS intentionally keeps nutrients barely detectable to bring out pastel and fluorescent coloration in SPS like Acropora. Classic examples include the Zeovit method (zeolite, bacteria and a suite of supplements) and similar commercial systems. ULNS tanks can look spectacular but are much less forgiving. Corals depend on careful feeding and amino acid supplementation, and alkalinity is usually kept lower (often ~6.5–7.5 dKH), since high alkalinity with very low nutrients is strongly associated with burnt tips. Most hobbyists, especially beginners, do better with moderate nutrients.
Nutrients and nuisance algae
Algae problems usually reflect a combination of excess nutrients, old or mismatched lighting, and low flow, plus not enough competition. High nitrate and phosphate feed green hair algae. Paradoxically, very low nutrients, especially when one is near zero, are linked to cyano and dinoflagellates. Read numbers in context.
Summary
Keep nitrate and phosphate low but measurable, roughly 2–20 ppm nitrate and 0.03–0.1 ppm phosphate for most reefs. Learn whether your kit reads phosphate or phosphorus. Reduce nutrients gradually with skimming, macroalgae, media or carbon dosing, and raise them by feeding or dosing if they bottom out. Change things slowly and watch your corals as closely as your test kits.
Frequently asked questions
What should nitrate and phosphate be in a reef tank?
Many mixed reefs do well around 5–20 ppm nitrate and 0.05–0.1 ppm phosphate, and SPS-dominant tanks often run somewhat lower. Keep both detectable rather than at zero.
Is zero nitrate bad for corals?
Often, yes. Corals' symbiotic algae need nitrogen, and zero nitrate is linked to pale corals and to dinoflagellate outbreaks.
How do I convert Hanna phosphorus ppb to phosphate ppm?
Multiply the phosphorus reading in ppb by 3.07, then divide by 1,000. For example, 30 ppb P is about 0.09 ppm PO4.
What is the fastest way to lower phosphate?
Granular ferric oxide or lanthanum chloride act quickly, but lower phosphate gradually because sudden drops can stress SPS corals.
Does a protein skimmer remove nitrate?
Not directly. It removes dissolved organics before they break down into nitrate and phosphate.
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More reef science guides
- Alkalinity, calcium & magnesium
- Nitrogen cycle & cycling
- Salinity & RO/DI
- Lighting & PAR
- Coral types & care
- Pests & diseases
- Reef conservation & science
- Coral symbiosis & Symbiodiniaceae
- Coral bleaching science
- Ultra-low nutrient research
- Ocean acidification & calcification
- Coral restoration & microfragmentation
- ICP testing & trace elements
Ranges are common hobby targets compiled from reef chemistry literature (including Randy Holmes-Farley's articles), hobby consensus and manufacturer documentation. They're not rules. Last reviewed 2026-10-15.