Nitrate & Phosphate: Target Ranges by Tank Type (and Why Stability Beats Zero)

Nitrate and phosphate get a bad reputation, but they aren’t waste to be eliminated — they’re the nitrogen and phosphorus every coral and every strand of algae in your tank needs to grow. The goal was never zero. It’s holding both nutrients in a range that suits what you’re keeping, and then holding that range steady week after week. Chasing zero, or letting either number swing wildly, causes far more problems than running a little “dirty” and stable.

Why “Zero” Nitrate and Phosphate Is a Mistake

Both nitrate and phosphate are required for coral tissue growth, photosynthesis, and pigmentation. A tank scrubbed down to truly undetectable nitrate and phosphate starves the very organisms you’re trying to grow — SPS in particular will go pale, lose tissue, and can die in an ultra-low nutrient system that isn’t being carefully supplemented to compensate. The target for almost every tank is low but measurable, not zero. Anything lower starves your corals; anything higher just feeds algae instead of them.

The Redfield Ratio: A Rough Guide, Not a Rule

You’ll hear reefers reference the “Redfield ratio” — roughly 16 parts nitrogen to 1 part phosphorus in marine plankton, which in nitrate-to-phosphate terms works out closer to 100:1 (for example, 5 ppm nitrate to 0.05 ppm phosphate). It’s a useful mental model for noticing when one nutrient is way out of proportion to the other, which often points to a specific cause — heavy phosphate from food with low nitrate export, for instance. But it’s not a target to chase precisely. Real reef systems vary widely, and matching your tank type’s range matters far more than hitting an exact ratio.

Target Ranges by Tank Type

• SPS-dominant: Nitrate 2–10 ppm, phosphate 0.01–0.03 ppm. SPS wants it lean; too high and colors muddy out, too low and tissue pales or recedes.
• Mixed reef (SPS, LPS & softies together): Nitrate 5–15 ppm, phosphate 0.03–0.10 ppm. A compromise range — SPS won’t show its absolute best color here, but nothing goes hungry either.
• LPS & soft coral tanks: Nitrate 10–20 ppm, phosphate 0.05–0.15 ppm. These corals tolerate — and often benefit from — dirtier water and faster growth at the higher end of that range.
• FOWLR (fish only with live rock): Nitrate under 20 ppm, phosphate under 0.10 ppm as a general ceiling. No coral coloration to protect, so the main concern is keeping algae in check and fish healthy.

Ways to Lower Nitrate and Phosphate

If you’re running above your tank type’s range, the fix is usually a combination of less input and more export rather than one silver bullet. Cut back on feeding first — it’s the most common cause of both nutrients creeping up. From there, a protein skimmer pulls dissolved organics out before they break down into nitrate and phosphate at all. GFO (granular ferric oxide) targets phosphate specifically and is a fast way to bring a spike down, though it needs regular replacement and can strip trace elements if overdosed. Carbon dosing (vinegar, vodka, or a product like NOPOX) feeds bacteria that consume both nitrate and phosphate as they multiply, and works best on an automated doser for consistency. A refugium growing chaetomorpha is one of the best long-term, low-drama nutrient sinks, since the algae is directly consuming what you’re trying to remove. Water changes help but are a slow, weak tool against nitrate and phosphate on their own — useful for trace elements and overall stability, not a fix for a real spike.

Why Stability Matters More Than the Number

This is the part that gets overlooked: there is no single perfect nitrate or phosphate number that works for every reef. Some tanks flourish at 15 ppm nitrate, others thrive nearer to 2 ppm — what matters far more is that the number doesn’t swing. Corals, and SPS especially, stress heavily when phosphate or nitrate jumps around, even if the average looks fine on paper. A tank holding a steady 10 ppm nitrate will almost always outperform one bouncing between 2 and 20 ppm, even though the second tank’s average is lower. Test consistently, track the trend rather than fixating on any single reading, and make changes to feeding or export gradually — sudden corrections create the exact instability you’re trying to avoid.

Pick your range, then hold it

Find the range that fits what you’re keeping — lean for SPS, a bit richer for LPS and softies, more forgiving for FOWLR — and then focus your effort on holding that range steady rather than hunting for a perfect number. Consistent testing, gradual adjustments, and real nutrient export (not just water changes) will get you there. Questions about dialing in your specific setup? Reach out, or browse our current WYSIWYG picks while you’re here.

Don’t Chase Zero

It’s tempting to treat undetectable nitrate and phosphate as the goal, but driving both to true zero often backfires — corals actually use some dissolved nutrients, and starving a tank of them can cause pale, slow-growing coral tissue and a crash in the very algae-eating clean-up crew you rely on. Aim for the low, stable end of your tank type’s target range rather than the absolute floor of your test kit.