Measurement
Two different water changes: calculate what remains
A worked example for unequal changes explains why adding the percentages gives the wrong answer.
By FishyMag · Published
Worked arithmetic example · Not a clinical review or treatment recommendation.
Quick answer
In the ideal mixed-water model, multiply the fractions left after each change. A 20% change followed by a 30% change leaves 0.8 × 0.7 = 56% of the original dissolved amount.
Work one change at a time
Start with a fictional dissolved amount of 100 units. Removing 20% leaves 80 units. After replacement and complete mixing, removing 30% of that amount leaves 56 units. The amount removed across the two steps is 44 units, not 50. The percentages apply to different starting amounts. No actual medication or target concentration is specified in this example.
State what the model leaves out
This assumes replacement water contains none of the tracked substance, each change removes the stated fraction and the water mixes fully between steps. It ignores binding to materials, chemical changes, filtration, evaporation and other additions. If any of those processes matters, the arithmetic does not describe the full system. A small predicted fraction does not prove a substance is absent.
Use the result as an explanation
Enter each change in order in the water-change planner and inspect the step-by-step table. Save the assumptions with the export. Keep a measured result in a separate record rather than replacing it with a calculated percentage. This tool cannot decide when to redose, combine products or return livestock.
Put it into practice
Your checklist
- Multiply remaining fractions in sequence.
- State replacement-water and mixing assumptions.
- Do not use the result as a redosing instruction.
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.
AI-assisted writing and development. Not hands-on product testing or veterinary review. How we publish →
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.