Evaporation top-ups vs water changes: which volume do you dose?

Two different water movements

A 100 L worked example

Suppose an operating system initially contains 100 L. If 3 L evaporates and is replaced, that is a 3 L top-up. If 20 L of aquarium water is removed and replaced, that is a 20 L water change. Recording “23 L water changed” would hide the difference.

Do not assume either event requires a full-system medicine addition. The medication water-change guide explains why the product’s instructions must select the procedure.

Salt is not replaced like evaporated water

NOAA explains that evaporation leaves salt behind. Replacing a pure evaporation loss with another salt dose would add salt that was not lost in that event. This does not account for salt physically removed by spills, wet equipment or other losses.

For a marine equipment example, Red Sea’s REEFER manual specifies RO water for its top-up reservoir. That instruction concerns the named system; follow your own system’s current manual and check salinity rather than transferring a target or correction rate from this article.

Read the salt-measurement guide. No therapeutic salt concentration is selected here, and salt behaviour is not a model for every medicine’s persistence.

Conditioner volume still depends on how it is used

Seachem Prime’s directions distinguish new water prepared separately from direct aquarium addition: the latter uses aquarium volume. Choose the applicable mode in the Prime calculator. A 3 L source-water batch and the system’s current water volume are different inputs, even when the batch replaces evaporation.

Source water must be appropriate for the animals and system. A medicine’s conditioner restriction does not make untreated disinfected tap water suitable. Resolve compatibility before selecting a preparation method.

A useful water record

• Starting operating level and actual or estimated system water volume.
• Volume lost to evaporation, recorded separately from removed water.
• Replacement source, preparation and amount actually added.
• Any salt, mineral, conditioner or medicine additions, each with its own reason and instructions.

Common questions

Does topping up replace medication removed by evaporation?

Evaporation does not establish how much of a particular medicine remains or has degraded. Do not create an evaporation-based medication top-up formula.

Can a top-up replace routine water changes?

It does not perform the same removal-and-replacement operation. This article does not select a maintenance interval for your aquarium.

What if I do both on the same day?

Keep separate measured entries and use the actual procedure with the relevant product instructions. Do not count the same water twice.

Sources and review scope

• NOAA sea-water explanation — salt remains as water evaporates
• Red Sea REEFER manual — named-system RO top-up reservoir instruction
• Seachem Prime directions — new-water versus direct-addition volume

Reviewed 28 October 2026. Examples and worksheets are DoseMyTank explanations, not clinical cases or independent product tests. Manufacturer directions apply to the exact formulation and situation described. Resolve a conflicting bottle label with the manufacturer before using a calculation.

Related practical guides

• GH and KH remineralisers: calculate a replacement-water batch