Measurement
Evaporation and top-off: a water balance is not a water change
An idealised example separates water loss from removal of a nonvolatile dissolved substance.
By FishyMag · Published
Worked arithmetic example · Not a clinical review or treatment recommendation.
Quick answer
In a model where only water evaporates and the tracked substance stays behind, concentration rises while its total amount stays constant. Adding clean replacement water restores volume; it does not remove that substance.
Separate the amount from the concentration
Imagine 100 mg of a fictional nonvolatile substance in 50 L. The calculated concentration is 2 mg/L. If only 5 L of water is lost, the same 100 mg in 45 L gives about 2.22 mg/L. Adding 5 L containing none of that substance returns the volume to 50 L and the modelled concentration to 2 mg/L.
Compare it with a real removal step
In the same ideal model, taking out 5 L of well-mixed water at 2 mg/L removes 10 mg. Adding clean replacement water then leaves 90 mg in 50 L, or 1.8 mg/L. Those operations differ because one removes the tracked substance and the other does not. The example does not describe every aquarium chemical: real substances may change, escape or interact with materials.
Write down the operation you performed
Use separate log descriptions for water removed, water added after removal and top-off after evaporation. Record the water source as well. Do not apply this example as a top-off recipe, a salt-management recommendation or a treatment adjustment. It is a way to prevent two different operations from disappearing behind the phrase “added water”.
Put it into practice
Your checklist
- Distinguish water loss from removal of mixed water.
- Keep substance amount separate from concentration.
- Log top-off and water replacement separately.
Sources and scope
This article distinguishes manufacturer statements from FishyMag’s own workflows and arithmetic examples. Linked formulation records retain their own evidence dates; publishing this article does not refresh those records.
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FishyMag reports what manufacturers say, quoted and linked. It does not diagnose illness or recommend a treatment, and it never infers that a product is safe from the absence of a warning — where no statement exists, it says so.
Formulations change. Always read the label on the bottle in your hand, and contact the manufacturer if your tank contains anything valuable or unusual.