Water changes during aquarium medication: do you replace the dose?
By Richard James · Manufacturer sources reviewed 28 October 2026
Replace medication only as the exact product's instructions direct. Some courses require a change before a full scheduled addition; others describe proportional replacement. A water change does not create a universal reason to add another full dose.
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Identify which situation you are in
- Scheduled preparation: a change explicitly required before an addition.
- An unscheduled change: maintenance or a water-quality problem during a course.
- End of treatment: removal or aftercare under the product instructions.
Record when the last addition occurred and which situation applies. Do not combine these three situations into one redosing rule.
Manufacturer examples: the rules differ
| Exact product | Published instruction | Implication |
|---|---|---|
| Ich-X | Hikari specifies at least a one-third change before each full treatment addition. | Read the complete timing and conditioner requirements; this is not a replacement-water-only amount. |
| PraziPro | Hikari specifies new water or a practical change of at least 30% when starting or restarting. | A mid-course change is not, by itself, a prescribed new treatment. |
| eSHa EXIT / eSHa 2000 | eSHa generally does not require routine pre/post changes, lists exceptions and describes proportional compensation for removed medication. | Read the specific product's instructions before calculating any compensation. |
| Cupramine | Seachem requires copper testing before redosing. | Volume arithmetic does not replace the measured copper level. |
Sources: Ich-X, PraziPro, EXIT, 2000 and Cupramine. This is a selected reference, not an exhaustive instruction sheet.
Why the percentage alone is not enough
In a hypothetical well-mixed 100 L system, removing 20 L removes 20% of whatever dissolved substance is present at that moment. It does not prove that 20% of the original medicine addition remains to be replaced: previous additions, removal media, elapsed time and the formulation can matter.
This example explains the difference between water arithmetic and a medication instruction. It is not a redosing formula. In particular, do not add both an improvised replacement amount and the next full scheduled amount.
Before the next addition
- Record the system water volume and the actual amount changed.
- Write down the exact bottle, last addition and course day.
- Check replacement-water preparation and conditioner restrictions.
- Read the product's change, testing and timing instructions together.
- Ask the maker when the situation is not covered.
Do not leave a serious water-quality problem unaddressed merely to preserve medication. Obtain appropriate fish-health advice for an unwell or deteriorating fish; this page cannot choose an emergency response.
Common questions
Should I automatically add a full dose after a 50% change?
No. Establish the applicable product instructions first. A scheduled full addition and proportional replacement are different actions.
Does DoseMyTank calculate a mid-course top-up?
The standard calculators scale their listed modes and courses. They do not reconstruct the amount of medicine currently in your water or choose an unscheduled top-up.
Missed a scheduled addition as well? Use the missed-dose checklist before changing the course.
Sources and review scope
- Hikari Ich-X directions — scheduled changes and additions
- Hikari PraziPro directions — starting/restarting a treatment
- eSHa EXIT FAQ — changes and compensation
- eSHa 2000 FAQ — changes and compensation
- Seachem Cupramine directions — testing before redosing
Reviewed 28 October 2026. Manufacturer statements are attributed above; examples and checklists are DoseMyTank explanations. This is not an independent test of efficacy, a diagnosis or a substitute for the supplied instructions. If your bottle differs, resolve the difference with its manufacturer before dosing.
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