Zero Nitrate and Phosphate: Why It’s a Problem

Why zero is a warning sign

Corals rely partly on the algae living in their tissue, and those algae need nitrogen and phosphorus. Published targets are all above zero for most reef tanks: Red Sea suggests about 2 mg/L nitrate and 0.1 mg/L phosphate for a mixed reef. Bulk Reef Supply warns that nutrient-stripping methods can starve a tank.

Pod Your Reef notes that tanks run with ultra-low nutrients often report dinoflagellate problems, and suggests keeping nitrate around 1–5 mg/L and phosphate around 0.03–0.1 mg/L.

Zero, or below what your kit can see?

Many kits can’t read the low end. A reading of “0” may mean 0.5 mg/L nitrate on a standard kit, or genuinely nothing. A low-range or high-resolution kit tells you which.

A tank full of algae can also test at zero, because the algae takes up nutrients as fast as they’re made. If you have visible algae and zero readings, the nutrients are there; they’re just in the algae. See hair algae.

Common causes

Zero readings usually follow a change that removes nutrients faster than before.

• Strong phosphate media such as GFO or lanthanum, especially when new
• Carbon dosing or biopellets
• A large refugium or algae scrubber
• Reduced feeding, or few fish for the size of tank
• A new, very efficient skimmer

Bringing them back up

Start with the gentlest change: reduce or remove phosphate media, ease off carbon dosing, or feed a little more. If that isn’t enough, some reef keepers dose a nitrate or phosphate supplement made for aquariums.

Go slowly and test every few days. Red Sea stresses gradual nutrient changes, because sudden shifts stress the algae inside corals. Aim to see a low but steady reading over two or three weeks.

Watch the ratio

Often only one runs out. Zero phosphate with nitrate still at 10 mg/L is a different problem from zero nitrate with plenty of phosphate. Test both and log them together so you can see which one is bottoming out.

Should I dose nitrate in a reef tank?

Some reef keepers do when nitrate is undetectable and corals look pale. Start with feeding changes, and dose only small amounts with regular testing.

Can phosphate be too low?

Yes. At zero, corals can pale and stop growing. Ease off phosphate removal first.

What causes dinoflagellates?

Often very low nutrients, among other factors. See our dinoflagellates guide.