Parameter Reference
PhosphatePO₄
Phosphate is half of the nutrient story. Without it, coral can't grow tissue; with too much, nuisance algae take over and calcification slows. The hobbyist target is 0.02–0.10 ppm — well above what surface ocean water carries (~0.005 ppm), because reef tanks aren't oligotrophic ocean and the bacterial uptake mechanisms differ.
Target ranges
| Beginner-safe | 0.02–0.15 ppm PO₄ |
| Optimal (consensus) | 0.02–0.10 ppm PO₄ |
| BRS recommended | 0.03–0.10 ppm |
| Triton target | <0.1 ppm |
| Natural seawater (surface) | ~0.005 ppm PO₄ |
Why it matters
Phosphate inhibits CaCO₃ crystal growth by binding to crystal surfaces — too much and coral can't calcify even with perfect Ca and Alk. Algae use phosphate as a primary growth nutrient, so high PO₄ feeds hair algae and bryopsis. Coral tissue and zooxanthellae need some PO₄ to grow normally — sustained zero starves them.
Symptoms of drift
Too low
Below 0.01 ppm: pale 'bleached' coral appearance, starvation stress, dinoflagellate dominance risk. The visual: SPS lose color saturation, then go pastel, then suffer tissue recession.
Too high
Above 0.10 ppm: algae growth (hair, bryopsis, cyano), inhibition of calcification, browning of coral tissue (excess zooxanthellae). Sustained 0.25+ ppm is fightable but the tank stays cosmetically rough.
Testing
Test kits divide by sensitivity. Hanna HI736 (LR, 0–2.5 ppm, 0.04 resolution) is fine for fixing a high-PO₄ problem. Hanna HI774 (ULR, 0–0.9 ppm, 0.01 resolution) is what you want for a healthy reef. Hanna HI713 (Phosphorus ULR, 0–200 ppb as P) is the highest precision but reports as P, not PO₄ — multiply by 3.066 to convert. Salifert and Red Sea PO₄ Pro titrations are usable but harder to read at the bottom of the scale.
Test weekly. PO₄ and NO₃ move differently — PO₄ binds to sand and rock and releases back over days, so changes lag behind feeding.
FAQ
- How do I lower PO₄?
- GFO (granular ferric oxide) in a reactor is fast and clean. Carbon dosing (NoPoX, vodka, vinegar) pulls both N and P together — slower but balanced. Lanthanum chloride works fast for emergencies but it's harsh. Reduce feeding as the upstream fix.
- Why does my PO₄ test read zero but I still have algae?
- PO₄ is bound up in algae tissue as it grows — the test only measures free, dissolved PO₄. A heavy algae load can hold the water column near zero while the substrate and algae mass store grams of PO₄ ready to release on disturbance. Phosphate is a flow, not a stock.
Sources & references
- 01Randy Holmes-Farley — Optimal Parametershttps://www.reef2reef.com/threads/optimal-parameters-for-a-coral-reef-aquarium-by-randy-holmes-farley.173563/
- 02Randy Holmes-Farley — Phosphorus and reefs (Advanced Aquarist)https://www.advancedaquarist.com/2002/9/chemistry
- 03Bulk Reef Supply — Choosing the right Hanna Phosphate Checkerhttps://www.bulkreefsupply.com/content/post/how-to-choose-the-right-hanna-instruments-phosphate-checker
- 04University of Washington — Seawater Mineralshttps://depts.washington.edu/embryology/index.php?id=mineral
- 05Triton — ICP testinghttps://www.triton.de/en/icp-testing
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