Most mixed reef tanks do well with nitrate somewhere between about 2 and 10 mg/L. Tanks dominated by small-polyp stony corals are often kept lower, under 1 mg/L. Aim for low but measurable, not zero, and judge your number by what your corals and algae are doing.
What the sources say
Published targets vary a lot, because they reflect different kinds of tank and different ideas about feeding corals.
| Source | Target (mg/L nitrate) | For |
|---|---|---|
| Red Sea | 2 | Mixed reef |
| Red Sea | 0.25–0.5 | SPS-dominated tank |
| Bulk Reef Supply | 3–5 | Most reef tanks |
| Bulk Reef Supply (algae article) | 5–10 | Tanks fighting hair algae |
| Holmes-Farley | Below 0.2 | Close to natural seawater |
Units are mg/L (ppm) of nitrate, NO₃.
Why not zero?
Nitrate is a nutrient. Corals rely partly on the algae living in their tissue, and those algae need nitrogen. When nitrate is driven to zero, many reef keepers see pale or brown corals and slower growth. Bulk Reef Supply warns that nutrient-stripping methods can starve a tank if they’re pushed too far.
Too much has the opposite problem. High nitrate feeds nuisance algae, and Holmes-Farley notes it can push the algae inside corals to grow too fast, which can slow the coral itself.
Read your kit’s unit
Most hobby kits report nitrate (NO₃) in mg/L or ppm. Some lab and pond kits report nitrate-nitrogen (NO₃-N). Nitrate is about 4.4 times the nitrate-nitrogen figure, so 1 mg/L NO₃-N is about 4.4 mg/L NO₃. Check the unit before you compare with any target.
Low-range kits are worth having if you aim below 2 mg/L. A standard kit may show “zero” when there’s still a little nitrate there.
Pick a target and judge it by results
Start from a published target for your kind of tank. Then watch what happens over a few weeks. If corals are colourful and growing and algae is under control, you’re probably fine, even if your number differs from someone else’s.
If nitrate is climbing or algae is spreading, see how to bring high nitrate down. Change things slowly: a sudden drop can stress corals as much as a rise.
Test regularly and look at the trend
Bulk Reef Supply suggests testing nitrate every 3–4 days in a new tank or after adding livestock, and every 1–2 weeks once it’s settled. Test phosphate at the same time, because the two usually move together. A steady line over weeks is the goal.
Common questions
Is 20 ppm nitrate too high for a reef tank?
It’s above most published reef targets, and algae is likely to take advantage. Bring it down gradually. Fish-only tanks tolerate more.
Is zero nitrate bad?
Often, yes. A reading of zero can mean corals are short of nutrients, or that algae is using it up as fast as it’s made. Look at the tank as well as the test.
Why is my nitrate high but phosphate low?
Nitrate and phosphate come from the same sources but are removed in different ways. One can run out while the other builds up. Track both.
Sources and limits
Last checked 12 October 2026. Ranges here come from the published sources below, which don’t all agree. They’re starting points to discuss with your own tank in mind, not guarantees for any coral.
- Red Sea: Algae Management Program manual
- Bulk Reef Supply: 5 ways to lower nitrates
- Bulk Reef Supply: getting rid of green hair algae
- Randy Holmes-Farley, Reefkeeping: Reef Aquarium Water Parameters
Keep going
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