Aquarium Dosing Calculator: Scale Label Doses to Your Tank

The formula

Every label dose is a rate: an amount per volume of water. To scale it:

Your dose = label amount × (your volume ÷ label volume)

Convert both volumes to the same unit first. The calculator converts between U.S. gallons and liters at 3.785 liters per gallon (NIST).

Worked examples

Which volume to dose

• Water conditioner during a water change: most labels say to treat the volume of new water. Some say to treat the whole tank. Follow yours.
• Fertilizers and treatments: usually dosed for the tank's water volume. Use the real volume, which our volume calculator works out after substrate and fill gap.
• Medications: label doses are usually per tank volume. Rock, wood and substrate displace water, so a heavily decorated tank holds less than you might think.

U.S. vs imperial gallons

An imperial (UK) gallon is about 4.546 liters, larger than the U.S. gallon. Products sold in the U.S. normally mean U.S. gallons. If a label is unclear, convert from the metric figure if one is given.

Measuring small doses

• Use a syringe or dropper marked in mL for liquid doses under 5 mL; household teaspoons vary.
• If the dose is below the smallest mark you can measure, it may be easier to premix in a measured amount of tank water, if the label allows.
• Weigh powders with a scale that reads to 0.1 g or finer, if the label gives grams.

Treat tap water

EPA states that chlorine and monochloramine must be neutralized or removed before tap water goes into an aquarium, and that products for this are available at aquarium supply stores. Choose a conditioner labeled for chloramine if your utility uses it. See our tap water guide.

Medications and safety

Read every part of the label before dosing a medication: species it's safe for, whether to remove carbon from the filter, effects on plants, snails and shrimp, repeat-dose timing and how long to treat. The FDA's consumer guide on ornamental fish drugs explains that many aquarium medications sold in stores and online are not FDA-approved or indexed, and describes how to recognize those that are. When fish are sick, a veterinarian with aquatic experience can help identify the cause before you treat.

Keep a record

Write down what you dosed, how much and when. If something goes wrong, the record helps you and anyone advising you work out why. It also makes repeat doses easy.

Common questions

How much conditioner for a 45 gallon water change?

If the label says 5 mL per 10 gallons, 5 × 45 ÷ 10 = 22.5 mL. Treat the volume of new water you add, unless the label says to dose the whole tank.

The label is in liters but my tank is in gallons. What do I do?

Choose liters for the label unit and gallons for yours; the calculator converts at 3.785 liters per U.S. gallon.

Should I dose for the tank's nominal size?

No. Measure the real water volume, which is often 10–20% less after substrate, the fill gap and decorations. Use the volume calculator first.

Can I round the dose up?

Small roundings are common for conditioners, but follow the label. For medications, measure as accurately as you can, since overdosing can harm fish, plants and invertebrates.