Aquarium dosing units explained: mL, mg, g, ppm and mg/L
By Richard James · Manufacturer sources reviewed 28 October 2026
Millilitres measure volume, milligrams and grams measure mass, and mg/L describes a concentration. They cannot be substituted without the relevant product and water information. Write the unit beside every quantity you calculate or measure.
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Start with the quantity, not just the number
| Unit | What it describes | Example |
|---|---|---|
| mL | Liquid volume. | 0.5 mL is one half of a millilitre, not 0.5 mg. |
| mg and g | Mass. | 1 g = 1,000 mg; 0.1 g = 100 mg. |
| mg/L | Mass of a specified substance per litre of solution. | 10 mg in a final volume of 20 L gives 0.5 mg/L. |
| ppm | A parts-per-million ratio; its basis matters. | For dilute water near 1 kg/L, mg/L and mass-based ppm are approximately equivalent. |
The EPA’s unit explanation describes the dilute aqueous assumption behind that ppm comparison. Do not transfer it automatically to concentrated products, gases or a solution with a materially different density.
Product volume is not active-ingredient mass
For an invented solution explicitly containing 10 mg of substance X per mL, adding 0.5 mL supplies 5 mg of X. If the final, evenly mixed volume is 20 L, the calculated added concentration is 5 ÷ 20 = 0.25 mg/L of X.
This is a maths example, not a medicine formulation or recommended concentration. It does not measure the amount remaining after adsorption, reactions, biological uptake or a water change.
A percentage needs its basis
- 1% mass/volume: if explicitly defined as 1 g per 100 mL, it equals 10 mg/mL.
- 1% mass/mass: 1 g per 100 g of product. Converting that to mg/mL requires the relevant density.
- A guaranteed analysis: identify what substance and reporting basis the label uses before calculating an addition.
Seachem’s Flourish FAQ specifically describes its guaranteed analysis as percentage by weight. Do not interpret every percentage as grams per 100 mL or as the concentration that one dose creates in the aquarium.
Keep the substance and test parameter visible
A reading needs the name of what was measured as well as its unit. A result expressed for an element and one expressed for a compound are not interchangeable simply because both say mg/L. Check the test instructions rather than comparing unlabeled numbers.
Likewise, GH and KH are separate water properties, and a product’s grams are not a hardness reading.
A quick result check
- Exact product and formulation identified?
- Amount unit preserved from the instructions?
- Water volume expressed in matching units?
- Substance or test parameter named?
- Amount per addition kept separate from concentration in water?
Common questions
Is 1 mL always 1 g?
No. That equality requires a density of 1 g/mL; do not assume it for an aquarium product.
Are ppm and mg/L always identical?
No. The dilute-water approximation has conditions. Check the source and the meaning of the ratio.
Can I use a spoon to measure milligrams?
Only with an applicable product-specific measuring instruction. Read the small-powder guide; a spoon’s volume is not a universal powder mass.
Sources and review scope
- US EPA unit-conversion explanation — dilute aqueous ppm versus mg/L assumption
- Seachem Flourish FAQ — guaranteed-analysis reporting basis
Reviewed 28 October 2026. Worked examples and worksheets are explanations, not clinical cases or independent product tests. Manufacturer instructions apply to the exact formulation and conditions described. Resolve conflicting pack instructions with the maker before using a calculation.