Red Sea NO3:PO4-X 5L
Pack size
500 mL, 1 L and 5 L
Cost per litre
Under 1c
Water type
Saltwater + Reef
Red Sea NO3:PO4-X 5L
The bulk size for multi-tank systems and the cheapest litre in the range. Red Sea NO3:PO4-X is a liquid carbon source that feeds the bacteria already living in your reef system, encouraging them to consume excess nitrate and phosphate straight out of the water column. This 5 L bottle is built for large display systems, fish rooms running several tanks off one dosing routine, or anyone who has already confirmed the product works and doesn't want to reorder every few months.
Key features
- Feeds your own bacteria — a blend of carbon sources, each favoured by a different naturally occurring bacterial strain, so nutrient reduction happens biologically rather than through media that needs replacing.
- Treats nitrate and phosphate together — one bottle addresses both nutrients that drive nuisance algae, instead of running separate products for each.
- The cheapest litre in the range — the 5 L bottle costs roughly 40% less per litre treated than the 500 mL starter size.
- Dosed by an exact chart, not guesswork — Red Sea publishes a daily dose in millilitres per 100 L, set by your current nitrate and phosphate reading.
- Room-temperature storage — no refrigeration required; keep out of direct light.
Specifications
| Spec | Red Sea NO3:PO4-X 5 L |
|---|---|
| Volume | 5,000 mL |
| Form | Liquid concentrate, dosed daily |
| Water type | Marine / reef |
| Litres treated at the conservative dosing rate | Approx. 166,650 L over the life of the bottle |
| Cost per litre of water treated | Under 1c per litre |
Dosing chart — Red Sea's published rates
Dose is set by your current nitrate and phosphate readings, not a flat daily amount. Test weekly while dosing and adjust as levels fall.
| Parameter | Current level | Daily dose |
|---|---|---|
| Phosphate (PO₄) | Below 0.04 ppm | 1 mL per 100 L |
| Phosphate (PO₄) | 0.04–0.2 ppm | 2 mL per 100 L |
| Phosphate (PO₄) | Above 0.2 ppm | 3 mL per 100 L |
| Nitrate (NO₃) | Below 1 ppm | 1 mL per 100 L |
| Nitrate (NO₃) | Above 1 ppm | 2 mL per 100 L |
Where both nutrients are elevated at once, follow Red Sea's full instructions on combining the two doses rather than simply adding them together.
Is the 5 L bottle actually better value?
Yes, and the numbers back it up rather than just the bigger pack looking like the deal. All three sizes land under 1 cent per litre treated, but the 5 L bottle is the cheapest of the three by a real margin.
| Size | Price | Approx. litres treated* | Cost per litre |
|---|---|---|---|
| 500 mL | $27.00 | ~16,650 L | Under 1c per litre |
| 1 L | $42.00 | ~33,300 L | Under 1c per litre |
| 5 L (this bottle) | $157.00 | ~166,650 L | Under 1c per litre |
*Calculated at Red Sea's top published dosing rate of 3 mL per 100 L per day — the conservative, lowest-yield case, used when phosphate reads above 0.2 ppm. At lower nutrient levels the same bottle treats considerably more water and lasts longer. Litre for litre, the 5 L bottle costs roughly 42% less than the 500 mL bottle and about 25% less than the 1 L bottle.
Suitable for, and not suitable for
- Suitable for large reef systems, sump-linked multi-tank setups, and anyone already dosing NO3:PO4-X who wants to stop reordering every few months.
- Suitable for use alongside Red Sea's Algae Management program when converting a tank off phosphate- and nitrate-absorbing media.
- Not suitable for use at the same time as sulphur-based denitrators.
- Not the size to start with if you haven't used this product before — try the 500 mL bottle first to confirm it suits your tank.
Dosage and directions
Add daily to the sump, or to a high-flow area of the display if there's no sump — never dosed directly onto coral. At this bottle size a dosing pump is close to essential, both for consistency and because the daily volume across a large or multi-tank system adds up.
Care and maintenance
Test nitrate and phosphate weekly while dosing and adjust to the chart above. Keep the protein skimmer running efficiently — the bacteria this product feeds are exported largely through skimmate, so a skimmer sized to your system matters as much as the dose itself.
Safety
Reduce the dose by half if nitrate falls below 0.25 ppm, and don't increase the dose faster than the chart allows — overdosing a carbon source can crash dissolved oxygen. Red Sea recommends switching off any UV steriliser while dosing, since UV will kill the bacteria the product is feeding. Not for use in planted or shrimp-only systems.
Goes well with
- Red Sea Nutrients Control Multi Test Kit (NO3/PO4) — the weekly nitrate and phosphate test this dosing chart is based on.
- Red Sea Reefer Skimmer 300 — efficient skimming is what exports the nutrient-laden bacteria this product feeds.
Warranty and delivery
Backed by Red Sea's standard product warranty, in addition to your rights under Australian Consumer Law. This is a 5 L bottle — expect a heavier parcel than the smaller sizes, though it still ships via standard freight.
Common questions
Is the 5 L bottle really cheaper per litre than the 1 L?
Yes — at $157.00 for 5 L against $42.00 for 1 L (which would cost $210.00 to match as five bottles), the 5 L bottle is roughly 25% cheaper litre for litre.
How long will a 5 L bottle last?
At Red Sea's top dosing rate it treats roughly 166,650 L of tank water. For a 1,000 L system dosed daily at the highest rate, that's around five and a half months; for a more typical mid-range dose it will run considerably longer.
Can I split a 5 L bottle across more than one tank?
Yes — dose each system separately according to its own nitrate and phosphate readings using the chart above; there's no need to combine systems onto one dosing schedule.
Should I buy the 5 L bottle if I've never used NO3:PO4-X before?
Start with the 500 mL bottle instead. It confirms the product and dosing rate suit your tank before you commit to a size that will take months to use up if something needs adjusting.
