Reef Nutrient Management: Balancing Nitrate and Phosphate

How to keep nitrate and phosphate in a reef tank low but not zero, why both excess and starvation harm corals, and the export methods that hold nutrients in balance.

Nitrate and phosphate are the two nutrients that most strongly shape the health of a reef aquarium. Both are needed in small amounts, yet both cause problems at extremes. Modern reef-keeping has largely abandoned the old goal of driving these values to zero, recognising that a low-but-detectable level keeps corals fed while still discouraging nuisance growth.

Why corals need some nutrients

Corals host symbiotic algae called zooxanthellae, which photosynthesise and pass simple sugars back to the coral as food. Nitrogen is a building block of the amino acids and nucleic acids those algae require, and phosphorus is central to ATP energy storage and to the phospholipids in cell membranes. When nitrate is driven to zero, the zooxanthellae are starved: hobbyists report that coral colour fades and growth tips become negligible. Conversely, no coral relies entirely on photosynthesis, so a steady trickle of nutrients supports tissue and skeletal growth.

What happens at the extremes

• Too high: excess nutrients fuel nuisance algae, cyanobacteria and diatoms, can brown out corals by overdriving zooxanthellae, and high phosphate is associated with reduced skeletal growth.
• Too low: corals starve, pale and may show slow tissue necrosis, and ultra-low-nutrient conditions are a common trigger for dinoflagellate outbreaks.

Target ranges

Natural reef water is extremely low in nutrients, but aquariums benefit from slightly higher, stable values. Practical targets that have been published for mixed reefs are roughly 5-10 ppm nitrate and 0.05-0.1 ppm phosphate; one editorial reports running tanks at about 8-10 ppm nitrate and 0.08-0.10 ppm phosphate. Wikipedia lists a working range of 0-10 ppm nitrate and 0-0.06 ppm phosphate. A survey of twenty successful hobbyists found phosphate averaging 0.073 ppm and nitrate averaging 8.64 ppm. The key idea is balance: the same editorial advocates keeping nitrate roughly 100 times the phosphate value, an aquarium echo of the Redfield ratio that links nitrogen and phosphorus in the sea.

Reducing inputs

The cheapest nutrient control is preventing inputs. Mix saltwater with reverse-osmosis deionised (RODI) water to keep metals and phosphate out, keep a modest fish population, rinse frozen foods before feeding to shed excess oils, and avoid overfeeding so uneaten food cannot decompose into the system.

Export methods