Phosphate (PO4) is the nutrient reefers love to hate. Too much feeds algae and can slow calcification; too little starves the corals' symbiotic algae and leads to pale, stressed corals. The goal is a stable, low but measurable value.
What phosphate level is ideal?
| Tank style | Typical PO4 target (ppm) |
|---|---|
| SPS, lower nutrients | 0.02–0.08 |
| Mixed reef | 0.03–0.10 |
| LPS / softies, heavier feeding | 0.05–0.15 |
Persistent readings above ~0.2 ppm usually come with algae or slower SPS growth. A true zero for weeks is a bigger danger to corals than 0.1 ppm (see why ultra-low nutrients can hurt corals).
Phosphorus vs phosphate: the unit trap
Some tests report phosphorus (P) in ppb, others phosphate (PO4) in ppm. Phosphate weighs about 3.07× phosphorus. Convert Hanna ULR phosphorus (ppb) to phosphate ppm: ppb × 3.07 ÷ 1000. Example: 30 ppb P ≈ 0.092 ppm PO4. Mixing these up is one of the most common reasons people chase phosphate they don't have.
How to test phosphate accurately
- Use a low-range colorimeter (e.g., Hanna ULR phosphorus or phosphate checker) or a quality low-range kit. Comparison: test kit guide.
- Rinse cuvettes with tank water, keep them fingerprint-free, and time the reaction exactly.
- Test at the same time of day; phosphate can dip during the photoperiod in algae-heavy tanks.
Why is my phosphate high?
- Heavy feeding (frozen foods, especially un-rinsed)
- Old or depleted RO/DI resin: check TDS is 0 (RO/DI guide)
- Phosphate leaching from new dry or used rock
- Detritus build-up in the sump or sand
How to lower phosphate
| Method | Speed | Notes |
|---|---|---|
| Water changes | Slow-moderate | Safe, but limited if source is ongoing |
| GFO (granular ferric oxide) in a reactor | Fast | Start with half the label amount; sudden drops can stress SPS |
| Lanthanum chloride | Very fast | Dose slowly, filter through a fine sock; overdose can harm fish |
| Refugium macroalgae (Chaeto) | Steady | Also lowers nitrate and helps pH |
| Carbon dosing | Moderate | Needs nitrate present too |
Lower by no more than roughly 0.05 ppm per day when corals are present.
How to raise phosphate
If phosphate reads zero: reduce or remove GFO, feed more, reduce carbon dosing, or use a reef phosphate supplement in small doses. Raise gradually and retest daily.
Sources
- Coral Compass: Nitrate and phosphate guide (with cited research)
FAQ
Is 0.00 phosphate good? No. Zero phosphate for long periods starves zooxanthellae and is linked to coral paling and bleaching. Aim for low but measurable.
Why does my phosphate read zero but I have algae? The algae may be consuming phosphate as fast as it is released. Test the algae's environment and reduce sources rather than dosing.
How fast does GFO lower phosphate? Within hours to days. Use less media than suggested at first to avoid shocking corals.