Reef tanks don't fail from "too much nutrients" as often as they fail from one nutrient near zero while the other stays elevated. Symbiodiniaceae and coral tissue need both nitrogen and phosphorus. When one is missing, growth and colour suffer and bleaching risk rises (research overview: ULNS research).
Practical targets
| Style | Nitrate (NO3) | Phosphate (PO4) |
|---|---|---|
| SPS lean | 1–5 ppm | 0.02–0.08 ppm |
| Mixed reef | 2–10 ppm | 0.03–0.10 ppm |
| Heavy LPS / softies | 5–20 ppm | 0.05–0.15 ppm |
A rough rule of thumb many reefers use: keep nitrate (ppm) roughly 50–100× phosphate (ppm). Example: at 0.05 ppm PO4, aim for roughly 2.5–5 ppm NO3. Exact Redfield ratios are for phytoplankton; aquarium targets are broader.
Common imbalance patterns
- NO3 high, PO4 near zero: often GFO or lanthanum overuse, or phosphate-hungry Chaeto. Pause phosphate export, feed more, or dose phosphate carefully.
- PO4 high, NO3 near zero: carbon dosing or a heavy denitrator with low fish load. Reduce carbon, feed more nitrogen (or stop over-exporting nitrate).
- Both high: feed less, clean detritus, water changes, refine export (phosphate, denitrification).
- Both near zero: riskiest for SPS colour. Feed, reduce export, raise both slowly.
How to correct without shock
Change one lever at a time. Retest every 2–3 days. Don't chase daily perfection; look at the trend across a week. Deeper dive: nitrate and phosphate science guide.
Sources
FAQ
Should nitrate be zero? No for most coral tanks. Softies and LPS often look better with measurable nitrate; SPS usually want low but not zero.
Can I dose nitrate and phosphate? Yes, with reef-specific supplements in tiny amounts. Raise slowly and retest.