To convert a Hanna HI736 phosphorus reading to phosphate, multiply the ppb number by 3.066 and divide by 1,000. That gives phosphate in ppm. A reading of 20 ppb phosphorus is 0.061 ppm phosphate, and a reading of 50 is 0.153 ppm. The Hanna HI713 already reads phosphate in ppm, so it needs no conversion.
Why does the Hanna HI736 read phosphorus instead of phosphate? #
The HI736 Ultra Low Range checker reports orthophosphate expressed as phosphorus (sometimes written PO4-P), in parts per billion. That’s a different unit from phosphate (PO4) in parts per million, which most reef targets and most test kits use.
The factor comes from atomic weights. A phosphorus atom weighs about 31 units. A phosphate ion is one phosphorus plus four oxygen atoms (4 × 16 = 64), for a total of about 95. So phosphate weighs 95 ÷ 31 = 3.066 times as much as the phosphorus inside it. Hanna explains the same conversion in its phosphate to phosphorus guide.
The formula:
ppm phosphate = ppb phosphorus × 3.066 ÷ 1,000
The divide-by-1,000 step converts ppb to ppm, since 1 ppm equals 1,000 ppb.
Hanna HI736 conversion table #
| HI736 reading (ppb P) | Phosphate (ppm PO4) |
|---|---|
| 1 | 0.003 |
| 5 | 0.015 |
| 10 | 0.031 |
| 15 | 0.046 |
| 20 | 0.061 |
| 25 | 0.077 |
| 30 | 0.092 |
| 40 | 0.123 |
| 50 | 0.153 |
| 60 | 0.184 |
| 80 | 0.245 |
| 100 | 0.307 |
| 150 | 0.460 |
| 200 | 0.613 |
The HI736’s range is 0-200 ppb phosphorus, which tops out at about 0.61 ppm phosphate. If your meter reads at or near the top of its range, the true value may be higher, and a low range phosphate checker is the better tool.
Converting the other way: ppm phosphate to ppb phosphorus #
If you know your target in ppm phosphate and want to know what the HI736 should read, multiply by 1,000 and divide by 3.066. Put more simply, multiply by about 326.
| Target (ppm PO4) | HI736 reading (ppb P) |
|---|---|
| 0.03 | 10 |
| 0.05 | 16 |
| 0.06 | 20 |
| 0.10 | 33 |
| 0.20 | 65 |
| 0.30 | 98 |
So a common SPS target of 0.03-0.10 ppm phosphate is roughly 10-33 on the HI736. Randy Holmes-Farley’s broader suggested range for mature mixed reefs, about 0.06-0.3 ppm in his nutrient target article, is roughly 20-98 ppb phosphorus.
HI736 vs HI713: which Hanna phosphate checker should you use? #
| HI736 (Phosphorus ULR) | HI713 (Phosphate LR) | |
|---|---|---|
| Reads | Phosphorus, ppb | Phosphate, ppm |
| Range | 0-200 ppb P (about 0-0.61 ppm PO4) | 0.00-2.50 ppm PO4 |
| Resolution | 1 ppb P (about 0.003 ppm PO4) | 0.01 ppm |
| Water type | Marine only | Fresh and marine |
| Conversion needed | Yes | No |
| Best for | Low-nutrient SPS tanks | FOWLR, freshwater, higher-nutrient reefs |
Hanna lists the HI713’s accuracy as ±0.04 ppm ±4% of reading at 25°C on its HI713 product page. That’s fine for a tank running 0.3 ppm but coarse for one aiming at 0.03, where the margin is bigger than the target. For tanks kept very low, the HI736’s finer resolution is the reason to own it.
Pick the HI713 if your phosphate usually sits above about 0.3 ppm, if you keep fish-only or freshwater tanks, or if you don’t want to convert units. Pick the HI736 if you run an SPS tank near the bottom of the range and need to see small changes.
Why do other phosphate readings not match my Hanna? #
- ICP lab results measure phosphorus as an element, including forms a checker can’t see. They’re often reported as P in µg/L (ppb) or mg/L (ppm). Check the unit before comparing, and expect some difference, because checkers read reactive orthophosphate.
- Color comparison kits have wide steps at low levels. A kit that reads “0.03” and a HI736 that reads 16 ppb (0.049 ppm) may both be reasonable.
- Cloudy samples read high on any colorimeter, since the meter measures light passing through. Particles from lanthanum dosing or stirred-up detritus can inflate a reading.
How do you get consistent HI736 readings? #
- Use clean cuvettes. Rinse with tank water, wipe the outside with a lint-free cloth, and hold them by the cap. Fingerprints scatter light.
- Fill to the 10 ml mark exactly, and zero the meter with that sample before adding reagent.
- Follow the reaction timing in the manual exactly. The color keeps developing, so reading early or late changes the result.
- Test at the same point in your routine, for example before feeding, so readings are comparable.
- Check it now and then with Hanna’s calibration check set, or compare against a second method.
How do you log HI736 readings without mixing up units? #
The unit mix-up is the classic mistake. You write “20” in a log that expects ppm, and a tank at a healthy 0.061 ppm now looks like a disaster.
Fish Tank Tracker stores phosphate in ppm and charts it against a 0-0.1 ppm band. Convert with the table above before logging, and put the raw meter number in the notes field, such as “HI736: 20 ppb.” That way the chart stays accurate, and you still have the original reading if you ever want to recheck the math. For more on testing habits, see how to test aquarium water. To act on a high number, read how to lower phosphate in a reef tank.
Frequently asked questions #
Is 10 ppb phosphorus good for a reef tank? #
Ten ppb phosphorus is about 0.031 ppm phosphate, which is low. It suits many SPS tanks. For a mixed reef it’s on the lean side, and you’d watch for pale corals.
What is 0.03 ppm phosphate on a Hanna HI736? #
About 10 ppb phosphorus. Multiply 0.03 by 1,000 and divide by 3.066 to get 9.8, which the meter displays as 10.
Can I use the Hanna HI736 in a freshwater aquarium? #
Hanna designed the HI736 for marine water, and its calibration assumes seawater. For freshwater, the HI713 is the model to use.
Why is my Hanna phosphate reading different from my Salifert test? #
They use different methods, resolutions and units. Convert both to the same unit first. Small differences at low levels are normal. For a direct comparison of Hanna and Salifert on alkalinity, see Hanna checker vs Salifert.
What does PO4-P mean on a test result? #
PO4-P means phosphate expressed as its phosphorus content. To turn a PO4-P value into phosphate (PO4), multiply by 3.066, keeping the same unit.