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Nitrate & Phosphate Dosing Calculator

Raise a measured nutrient with a known-strength solution. Check the salt, the hydrate and the test units before turning a reading into a dose.

True system water

Starting readings and targets are examples, not recommended reef levels. Confirm the test’s range and repeat an unexpected result.

Assay / purity optional
Split the correction across days optional
Check the current N:P ratio optional

Known strength, measured water

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Enter your readings and nutrient source.

Save stores this nutrient’s stock recipe or bottle strength on your active tank. Current readings and targets are not saved.

How much nitrate or phosphate should I add to a reef tank?

Calculate the gap between a reliable reading and a target you have chosen for your system. This tool converts that gap into an amount of a known compound or solution. It does not diagnose a pale coral or choose a nutrient target from the appearance of algae. Check both nitrate and phosphate, the test’s reporting units, and the actual water volume before calculating a correction.

For an illustrative 100 L system going from 1 to 5 ppm NO₃, the gap is 4 ppm and the required nitrate is 400 mg. Pure sodium nitrate supplies that in 0.5483 g. A stock made from 10 g, topped up to a final 1,000 mL, needs 54.83 mL. The amount added is predictable; the later test reading also depends on what the aquarium consumes or removes.

Use true system water, including the sump and excluding displacement. If the target is already met, no corrective addition is calculated. If it is lower than the current reading, use the reef water-change calculator to explore dilution. Adding a nutrient cannot remove it.

Which nitrate or phosphate salt am I actually weighing?

A chemical name without its hydrate can be incomplete. Calcium nitrate contains two nitrate groups; its tetrahydrate also carries four water molecules. Sodium phosphate monobasic is sold without crystal water, with one water, or with two. Those waters add weight without adding nitrate or phosphate. The formula on the container must match the selection.

Hydrates change the recipe. For equal nitrate, calcium nitrate tetrahydrate needs 1.439× the mass of the anhydrous compound. The theoretical pure trisodium phosphate dodecahydrate needs 2.319× the anhydrous mass for equal phosphate. “Close enough” here can mean a very different dose.

Trisodium phosphate needs an extra identity check: JECFA describes a crystalline form containing additional sodium hydroxide. A cleaner or free-alkali mixture is outside the pure-form calculation. Likewise, Yara’s agricultural calcium nitrate contains ammonium nitrogen as well as nitrate nitrogen. A percentage on a fertilizer bag does not make it a pure Ca(NO₃)₂ preset. Unknown “xH₂O” water content also cannot be fixed with a purity slider.

How do I mix a nitrate or phosphate stock solution?

Weigh the identified reagent, dissolve it in part of the RO/DI water, and then top up to the stated final solution volume. Adding a powder to a full liter of water is not the same instruction as making a liter of finished stock. Label the bottle with the formula, hydrate, grams, final volume and preparation date. Follow its SDS for handling and storage; nitrate salts are oxidizers.

In the tool, “I know the grams” checks a stock you have specified. “Design a stock” works backward from the rise you want one measurable liquid dose to provide. A phosphate example shows why stock dosing helps: moving 100 L from 0.01 to 0.05 ppm PO₄ needs only 0.005053 g of pure anhydrous NaH₂PO₄. A 1 g / 1,000 mL stock makes that a 5.05 mL dose.

Assay basis matters as much as the percentage. “On an anhydrous basis” and “as weighed” are different claims for a hydrate. Use the purity field only for the mass fraction of the exact selected compound in the material weighed. It cannot account for an unknown hydrate or certify contaminants as aquarium-safe.

This planner is limited to DIY stocks at or below 100 g/L and liquid additions no larger than 1% of system volume. These are calculation limits, not solubility or livestock-safety guarantees. Use weaker stocks when necessary, confirm everything dissolves, and keep calcium and phosphate concentrates separate to avoid precipitation.

How do I calculate a NeoNitro, NeoPhos or other liquid dose?

Choose the commercial-liquid option and copy the bottle’s statement in the form “this many mL raises this much water by this many ppm.” Keep the label’s water units separate from your tank’s display units. If the concentration is reported as nitrogen or phosphorus, select that reporting basis too.

Brightwell NeoNitro and NeoPhos instructions say that different strengths have been sold. This page therefore does not silently substitute a historical formula. A routine like “dose twice weekly” is not enough to infer potency, and the proprietary salt mixture does not reveal its sodium, potassium or calcium contribution. The tool reports those co-ions as unknown for a commercial liquid.

Is my checker showing phosphate, or phosphorus?

Read both the substance and the unit. A phosphate result in ppm PO₄ is not a phosphorus result in ppb P. Hanna’s HI736 reports ppb phosphorus; its official conversion multiplies by 3.066 and divides by 1,000 to obtain ppm phosphate. Using full formula masses, 10 ppb P becomes 0.03066 ppm PO₄ here.

The input selector converts nitrate-N to nitrate and orthophosphate-P to phosphate before calculating a dose. Changing the selector preserves the entered concentration while expressing it in the new units. It does not turn a total nitrogen or total phosphorus measurement into a nitrate or orthophosphate measurement; those tests can include other chemical forms.

How quickly can I raise nutrients, and what else does the dose add?

Enter a daily addition only after choosing a pace suitable for your system and product. The tool divides the correction into portions and a smaller final dose where needed. At an illustrative 1 ppm NO₃ per day, the 100 L example takes 4 additions of 13.71 mL. That is a mass budget under fixed conditions, not a promise that the tank reaches its target on that date.

Follow the manufacturer’s timing and retest before continuing; Brightwell’s linked instructions include a 24-hour recheck. A falling or unchanged reading is evidence to investigate, not a command to add the remaining budget blindly. The stock and the tank may also have changed since the first measurement.

Choosing potassium nitrate for the same 4 ppm correction adds 2.522 ppm potassium. Either pure calcium nitrate form adds 1.293 ppm calcium; the hydrate changes the weighed grams, not the nitrate-to-calcium ratio. Sodium nitrate avoids those K and Ca additions but adds sodium. None of these is a complete reef or planted-tank nutrition plan.

Why did phosphate barely rise after I dosed it?

First check the test units, true volume, stock strength and whether the reagent dissolved. Then consider removal: laboratory experiments reported by NOAA show phosphate adsorption onto calcite and aragonite. Aquarium rock and sand can therefore help explain why the measured water-column increase is smaller than the calculated addition. That observation does not tell this calculator how much your particular rock will bind.

Uptake, filtration and other additions also affect the result. Repeat the measurement with the same method and timing, record what actually happened, and recalculate from the new reading. Treat a zero reading as below that test’s ability to resolve the nutrient, rather than proof of exactly zero molecules. For ongoing automatic water exchange, check the replacement schedule as another part of the nutrient budget.

Should I dose to the Redfield N:P ratio?

The quotient of nitrate ppm and phosphate ppm is a ratio of ion masses. It is neither elemental N:P by mass nor N:P by moles. Entering 5 ppm NO₃ and 0.05 ppm PO₄ gives 100:1 ion mass, 69.27:1 elemental mass, and 153.17:1 molar N:P. A 16:1 molar ratio would correspond to about 10.45:1 NO₃:PO₄ by mass.

A ratio conversion is not a dosing target. Nitrate does not represent every nitrogen source, and these tests do not measure total nutrient availability. Experimental coral research links nutrient imbalance and phosphate limitation to stress, but does not establish a universal aquarium ratio. Check each concentration and the tank’s response independently. For complete planted-tank recipes, use EI / PPS dry dosing or the liquid fertilizer comparison.

If nutrients are persistently high instead, the reef carbon dosing calculator compares vodka and vinegar and scales a conditional protocol. Carbon additions support nutrient removal; they do not follow the predictable dose-to-rise math used here.

Formula sources and product limitations

Formula masses use CIAAW atomic weights. The exact reagent identity links below were checked October 31, 2026. They establish formula and hydrate, not blanket approval of a product for aquarium use. Match the container’s identity, assay basis and SDS before making a stock.