Water
Water changes without a new water problem
A controlled partial change can remove dissolved waste, but replacement water must be suitable for the fish.
Prepare the replacement
Check temperature and relevant water chemistry. Use a conditioner or treatment system appropriate to the supply and follow its instructions for chlorine and chloramine. Know the actual water volume being treated.
Make the work predictable
Keep intakes safe, protect fish from sudden level drops and make sure pumps cannot run dry. Stay with the process; an unattended hose can overflow a pond or change conditions far more than intended.
After the change
Confirm normal level, flow and aeration. Record the amount changed and retest if the change was responding to a water-quality problem. The cause still needs to be found.
Avoid a universal percentage
The right amount depends on the problem, fish and difference between old and replacement water. Very large sudden changes need particular care. Seek urgent help when fish are distressed rather than relying on a generic routine.
Create a repeatable routine
Record your estimated system volume, how much water you replace, the product used to condition it and any maintenance carried out at the same time. Label the fill and drain controls clearly.
Sources & further reading
Original WakeyKoi wording, informed by the references below. Linked clinical resources explain the evidence and their scope; they have not reviewed or endorsed this website.
Related guides
Educational guidance cannot diagnose an individual fish. Breathing distress, severe injury or unexplained losses need prompt professional assessment.