Dosing from a label: scale the ratio, never the guess

The label states a ratio, not a dose

Almost every aquarium product expresses itself as an amount per a stated volume of water: so much per so many gallons or litres. That is a ratio, and it only becomes a dose once it is multiplied by the water your aquarium actually holds. Everything that goes wrong in dosing happens in that multiplication.

Copy the ratio from the label in front of you rather than from memory or from a forum post. Manufacturers reformulate, concentrations differ between a standard and a concentrated version of the same brand, and regional versions of a product are not always identical. The label on the bottle in your hand is the only authority for that bottle.

Use the real water volume

Dose against the water the aquarium actually contains, not the size printed on the tank. Substrate, rock, wood and equipment all displace water, and freeboard means the tank is never filled to its nominal capacity. Dosing a "twenty gallon" tank that really holds meaningfully less is an overdose, every time, by the same proportion.

Work out the real figure once and keep it recorded; the water volume guide covers how to measure it and what to subtract. If the system includes a sump or connected vessels, decide deliberately whether the treatment applies to the display volume or the whole system volume, because they are different numbers.

Units are where the errors hide

Keep the label's own units all the way through the calculation, and only convert when you must. Millilitres and teaspoons, gallons and litres, US gallons and imperial gallons, drops and millilitres: each conversion is an opportunity for a factor-of-something mistake, and aquarium doses are small enough that a single slip matters.

Measure with something graduated in the unit you are working in, rather than estimating with a cap or a kitchen spoon. For small volumes, a syringe is more accurate than a cap. Fishician's label dose calculator scales the ratio you enter to the volume you enter and keeps the units visible, so the arithmetic can be checked rather than trusted.

What the arithmetic cannot tell you

A correct calculation says nothing about whether the product is appropriate. It does not confirm the diagnosis, the suitability for your species or invertebrates, interactions with anything else in the water, the effect on the biofilter, maximum safe dose, repeat frequency, or whether the product is safe with the animals you keep. Many treatments are harmful to invertebrates, some are hard on nitrifying bacteria, and a few should never be combined.

Read the whole label, including contraindications and warnings, before dosing anything. If you are treating a health problem, check water quality first: environmental causes are far more common than the diseases people reach for medication to treat, and the illness signs guide explains why visible signs rarely identify a specific cause. Severe, urgent, persistent or uncertain problems belong with a qualified aquatic veterinarian.

Dose deliberately, and write it down

Add treatments where there is good circulation so they disperse rather than sitting in one corner, and avoid dosing into a filter intake basket or directly onto animals. If a product requires removing chemical filtration such as activated carbon, that instruction exists because the carbon would otherwise strip the product out; follow it and remember to put it back afterwards.

Record the product, the dose, the volume it was calculated against, the time and the reason. A course of treatment usually has a schedule, and the record is what keeps the second and third doses correct. It is also what lets you, or a vet, understand what happened if the aquarium responds badly. The maintenance guide covers keeping that history usefully.

Sources and further reading

Each source is listed with the part of this guide it supports. Source scope matters: a taxonomy record, product label, or general husbandry page does not automatically support every care claim.

• Merck Veterinary Manual: Routine Health Care of Fish
  Treatment practice, why environmental causes are checked first, and when professional diagnosis is required.
• Merck Veterinary Manual: Management of Aquarium Fish
  Water-quality management, biofilter protection, and aquarium-management context for treatments.
• University of Florida IFAS: Fish Health Management in Recirculating Systems, Part 1
  System water-quality management and the effect of treatments on nitrifying biofiltration.