Nutrient Export in a Reef Tank: Controlling Nitrate & Phosphate

Nutrient export in a reef tank means removing nitrate and phosphate as fast as food and fish waste add them. The main tools are water changes, a protein skimmer, a refugium growing chaeto, phosphate removers such as GFO, and carbon dosing. In a mixed reef, aim for roughly 2–10 ppm nitrate and 0.03–0.1 ppm phosphate, never zero.

This guide balances the equation: where nutrients come from, every practical export method compared, target levels for each type of tank, how to set up a simple refugium and how to fix high nitrates or, just as important, nutrients that have bottomed out.

Updated October 2026 by the team at Natural Seawater Supplier, a Croydon, London-based supplier of live natural seawater for marine aquariums, delivering across the UK 7 days a week.

Key takeaways

• Food is the biggest nutrient input in almost every marine aquarium; most nitrate and phosphate problems start with feeding.
• Mixed reef targets are about 2–10 ppm nitrate and 0.03–0.1 ppm phosphate; SPS tanks run lower and fish-only tanks can tolerate nitrate up to about 40 ppm.
• Zero is not the goal: nutrient-starved tanks show pale corals and are prone to dinoflagellate outbreaks.
• Test the water you add: RO/DI should read 0 TDS, and natural seawater typically contains just 0–0.9 ppm nitrate and 0–0.05 ppm phosphate.
• A refugium lit on a reverse-daylight schedule exports nutrients and helps steady pH overnight.

Nutrients in vs nutrients out

Think of your tank as a bank account. Nitrate and phosphate build up whenever inputs exceed exports. Excess nutrients fuel nuisance algae and, as research on wild reefs shows, nutrient pollution reduces coral calcification. Balance the two columns and the tank stays clean.

Nutrients in	Nutrients out
Fish food, especially frozen and pellets fed in excess	Water changes
Fish and invertebrate waste (bioload)	Protein skimming
Top-off water (poor RO/DI or tap water)	Macroalgae in a refugium (chaeto)
Decaying detritus, dead livestock, uneaten food	Phosphate removers (GFO, lanthanum)
Coral foods and amino acid additives	Carbon dosing (vodka, vinegar, commercial products)
Low-quality salt or source water	Algae turf scrubbers; coral and coralline growth; filter socks

Food: the biggest input

Everything you feed adds nitrogen and phosphorus. Higher-quality foods are digested more efficiently, so less becomes waste, but any food overfed ends up as nitrate and phosphate. Thaw and rinse frozen food, feed only what is eaten in two to three minutes and see our guide on how much and how often to feed marine fish.

Top-off and change water: the hidden input

Many hobbyists chase high nutrients for months before testing their RO water and finding it full of nitrate and phosphate. Check your RO/DI with a TDS meter (it should read 0) and replace cartridges and resin when it rises. Test any saltwater you add too. Nutrients on natural reefs, and in natural seawater, are close to nil, so anything that pushes your levels well above that is worth tracking down.

What should nitrate and phosphate be in a reef tank?

System	Nitrate (ppm)	Phosphate (ppm)
Fish only / FOWLR	Under 20–40	Under 0.5
Soft coral reef	5–20	0.05–0.2
Mixed reef (LPS + some SPS)	2–10	0.03–0.1
SPS-dominant reef	1–5	0.02–0.08
Ultra-low nutrient system (ULNS)	Under 1	Under 0.02

These are hobby consensus ranges. Stability matters more than hitting an exact figure, and many reefers find a nitrate-to-phosphate ratio of roughly 100:1 keeps algae and cyanobacteria at bay. Our marine tank water parameters guide covers the other numbers to test alongside them.

Nutrient export methods compared

Method	Removes	Cost	Effort	Best for
Water changes	Nitrate, phosphate, organics	Low per change	Weekly	Every tank
Protein skimmer	Organics before they break down	£200–£450	Low	Every reef over about 100 L
Refugium with chaeto	Nitrate and phosphate	Low	Monthly harvest	Tanks with a sump
GFO / lanthanum	Phosphate only	Low–moderate	Media changes	Stubborn phosphate
Carbon dosing	Nitrate and phosphate (via bacteria)	Low	Daily dosing, close testing	High nitrate, experienced keepers
Algae turf scrubber	Nitrate and phosphate	Moderate	Weekly scraping	Heavily fed fish tanks

Water changes

The simplest export: a 10% weekly change removes 10% of the nitrate and phosphate and replaces trace elements. Its effect depends on the water you add being nutrient-free. See how much and how often to change water for the right schedule.

Protein skimmer

A skimmer removes dissolved organic waste before bacteria convert it to nitrate and phosphate. Size it for your real water volume and empty the cup at least weekly; a wet, overflowing cup removes water but little waste.

Refugium and chaeto

Chaetomorpha (chaeto) is a fast-growing macroalgae that locks up nitrate and phosphate as it grows. Each time you harvest it, those nutrients leave the system. A refugium also breeds copepods and amphipods, natural live food for fish and corals.

GFO and phosphate removers

Granular ferric oxide (GFO) binds phosphate. Run it in a reactor and start with half the recommended dose: dropping phosphate too quickly can shock corals. Lanthanum chloride works faster but must be dosed slowly and filtered through a fine sock.

Carbon dosing

Dosing an organic carbon source (vodka, vinegar or a commercial product) feeds bacteria that consume nitrate and phosphate, which the skimmer then removes. Start at a quarter of the recommended dose, increase slowly and test nitrate twice a week. Overdosing can cause bacterial blooms and red slime algae (cyanobacteria). A working skimmer is essential.

Algae turf scrubber (ATS)

An ATS grows turf algae on a brightly lit screen outside the display. Scrape it weekly and the nutrients leave with the algae. It suits heavily fed fish tanks where chaeto alone cannot keep up.

How to set up a refugium

1. Use a sump chamber of at least 10–15% of the system volume, with a filter sock or sponge upstream to catch detritus.
2. Set gentle-to-moderate flow so the chaeto ball slowly tumbles; dead spots let it rot from the centre.
3. Light it with a 6,500K or dedicated refugium LED giving roughly 80–150 PAR at the algae.
4. Run the refugium light on a reverse-daylight schedule, on when the display lights go off, for 12–16 hours.
5. Start with a fist-sized ball of chaeto and let it fill the chamber.
6. Harvest about half whenever it fills the space, typically every two to four weeks, and throw the trimmed algae away.

The reverse schedule matters: photosynthesis in the refugium consumes carbon dioxide at night, when the display’s corals are releasing it, so pH swings less. The main lighting plan is covered in our reef tank lighting schedule guide.

How to lower high nitrates in a saltwater tank

1. Test the RO/DI and the saltwater you add; fix any source first.
2. Cut feeding back by a third and rinse frozen food.
3. Siphon detritus from the sand and behind rockwork, and change filter socks every three to four days.
4. Do 15–20% water changes weekly for three to four weeks with nutrient-free water.
5. Check that the skimmer is producing dark, dry skimmate.
6. Add a refugium or start carbon dosing if nitrate still sits above your target.
7. Lower nitrate gradually, by no more than about a third in a week, so corals can adjust.

The danger of zero nutrients

Chasing undetectable nitrate and phosphate is one of the most common mistakes in modern reefkeeping. Corals need some nitrogen and phosphorus to feed their zooxanthellae. When both read zero, corals pale or turn pastel, polyp extension drops and growth stalls. Bottomed-out tanks are also prone to dinoflagellates, a stubborn brown, snotty film that thrives where competing algae and bacteria are starved; our saltwater algae identification guide shows how to tell dinos from diatoms and cyano.

Ultra-low nutrient systems (ULNS) deliberately run nitrate under 1 ppm and phosphate under 0.02 ppm for intense SPS colour. They work for experienced keepers who test frequently and feed corals directly. Most hobbyists get better results in the mixed reef ranges above. If your tank has crashed to zero, reduce export first (less GFO, less carbon dosing, a shorter refugium photoperiod), feed a little more and, if needed, dose nitrate or phosphate to reach the bottom of your target range.

For water changes that dilute nutrients rather than adding them, natural seawater typically tests at 0–0.9 ppm nitrate and 0–0.05 ppm phosphate. Get a quote for natural seawater delivery to your door, seven days a week.

Frequently asked questions

What is nutrient export in a reef tank?

Nutrient export is any method that removes nitrate and phosphate from a reef tank. The main methods are water changes, protein skimming, a refugium with chaeto, phosphate removers such as GFO, carbon dosing and algae turf scrubbers.

What should nitrate and phosphate be in a reef tank?

For a mixed reef, aim for about 2 to 10 ppm nitrate and 0.03 to 0.1 ppm phosphate. SPS-dominant tanks often run lower, around 1 to 5 ppm nitrate, while fish only tanks can tolerate nitrate up to about 40 ppm.

What causes high nitrates in a saltwater tank?

High nitrate is usually caused by overfeeding, too many fish for the system, trapped detritus or nitrate in the top-off or change water. Check your RO/DI with a TDS meter, cut feeding back and increase water changes and skimming.

How long should a refugium light be on?

Run a refugium light for 12 to 16 hours a day on a reverse-daylight schedule, switching on when the display lights go off. This keeps chaeto growing and helps stabilise pH overnight.

Is chaeto better than GFO?

Chaeto removes both nitrate and phosphate and supports copepods, while GFO removes only phosphate. Chaeto is cheaper to run but needs a sump and light; GFO acts faster on stubborn phosphate. Many reefers use chaeto and add GFO only when needed.

Can nitrate and phosphate be too low in a reef tank?

Yes. When nitrate and phosphate read zero, corals often pale and stop growing, and the tank becomes prone to dinoflagellates. Reduce export, feed a little more or dose nutrients back into the target range.

Bottom line

Good nutrient export in a reef tank is about balance, not elimination. Control inputs first by feeding sensibly and adding clean water, then remove the rest with water changes, a skimmer and a chaeto refugium, adding GFO or carbon dosing only when needed. Keep nitrate and phosphate low, stable and detectable, and corals will out-compete algae.

Sources

External sources cited in this article (checked October 2026):

• Nutrient pollution reduces coral calcification (NOAA)