compoundPO₄³⁻mg/L
Phosphate
Phosphate is essential for plant growth, and heavily planted tanks are routinely dosed with it. The idea that phosphate causes algae is a simplification: algae need it, but so do plants, and the limiting factor in most algae outbreaks is light or carbon rather than phosphate.
In one sentence
Phosphate is a plant nutrient that accumulates from food and waste. In planted tanks it is dosed deliberately; in unplanted tanks it is often blamed for algae with less justification than it deserves.
How to read a measurement
Bands are guidance for interpreting a reading, not thresholds with sharp edges. Sensitivity varies by species and by how long the exposure lasts.
| Band | Range | Verdict | What it means |
|---|---|---|---|
| Plant-limiting | ≤ 0.05 | watch | Low enough to limit plant growth in an injected tank. In a planted aquarium this is a deficiency rather than an achievement. |
| Normal planted range | 0.05–2 | target | Where planted tanks are deliberately dosed. Phosphate is a plant nutrient, and the widespread belief that it must be driven to zero comes from reef practice, where the chemistry and the organisms are different. |
| Elevated | 2–5 | acceptable | Not directly harmful to fish at aquarium concentrations. Usually indicates accumulating organic load or a phosphate-rich water supply, and it is that underlying cause rather than the phosphate itself that is worth addressing. |
| High | ≥ 5 | watch | Points at a maintenance problem — overfeeding, accumulated detritus, or a supply high in phosphate. Phosphate is not the reason a tank has algae; light and available nitrogen usually are. Stripping phosphate from a planted tank to fight algae removes a nutrient the plants need and hands the advantage to the algae. |
Where it comes from
- Fish food, which is the dominant source in most tanks.
- Tap water in some supply areas, where phosphate is dosed to reduce lead pickup from old pipework.
- Decomposing organic material in the substrate.
How it is measured
- Liquid reagent test kit, which measures orthophosphate and not phosphate bound in organic matter
- Test strips, which are too coarse to distinguish the bands above
Typical aquarium range
- Typical
- 0.5–2 mg/L