Aquarium medication is usually dosed by water volume. The advertised tank capacity overstates actual water because glass thickness, air space, substrate, rocks and wood displace volume. Use internal dimensions and actual fill height for a better estimate, then account conservatively for major displacement. If a label says a fixed amount per litre or gallon, calculate from net volume and measure the dose accurately.
A 100-litre aquarium rarely contains 100 litres of treatment water. That difference matters most with medications that have a narrow safety margin.
Start with internal water dimensions
Use the inside length and width of the tank and the actual water depth rather than the external dimensions or nominal model name. The tank volume calculator can do this conversion for rectangular aquariums.
Irregular tanks need a manufacturer figure or a measured fill approach.
Account for displacement
- Substrate occupies part of the tank volume.
- Large rocks and driftwood displace water.
- Water may sit several centimetres below the rim.
- Internal filter chambers can change how much water is in the display.
Follow the label unit exactly
Check whether the product is dosed per litre, UK gallon, US gallon or a fixed tank volume. Do not silently swap units.
Use a syringe or graduated measure when small differences matter.
Redosing after water changes
Some medications are redosed after a specified percentage water change; others follow a day-by-day schedule. Do not simply add a full dose after every change.
Record the amount of water removed and follow the manufacturer's instructions.
Monitor the tank during treatment
Test ammonia and nitrite because illness and some treatments can stress the biological filter. Observe oxygenation, appetite and behaviour and stop or seek advice if the product label identifies adverse reactions.
Related aquarium tools and guides
Frequently asked questions
Should I dose for the tank's advertised litres?
Not if the actual water volume is clearly lower. Net volume gives a better concentration estimate.
Do rocks and substrate really matter?
They can, especially in heavily aquascaped tanks or with concentrated medicines.
Can I round the dose up?
Do not exceed the label concentration simply for convenience. Measure as accurately as practical.
Sources and further reading
This guide separates general aquarium arithmetic from fish-welfare decisions. For species-specific limits, always check the needs of the fish you intend to keep.