Water Changes: Define the Job Before Choosing a Volume

A water change can replace part of the aquarium water, dilute some dissolved substances and replenish others. It does not automatically identify or remove the cause of a problem.

Be clear about the purpose

Routine maintenance, correction of a verified contaminant and managing an unexpected chemistry result are different situations. The useful volume and frequency depend on that purpose, the existing water and the replacement water. There is no universal percentage in this guide.

Prepare replacement water deliberately

Use appropriate purified source water and a marine salt mix, following its mixing instructions. Verify salinity and temperature as appropriate before use. A mixing container's appearance or an estimated scoop count does not replace measurement.

Understand the limit

A partial change does not remove a substance's continuing source or resolve failed equipment. It also changes more than one component of water chemistry. If animals are declining, document readings, maintenance and additions while seeking informed advice.

Keep an operation record

• Note the amount changed and why.
• Record replacement-water measurements.
• Check equipment operation afterward.
• Compare subsequent readings without assuming a change proves a diagnosis.

For a suspected emergency

Promptly obtain advice appropriate to the contaminant or failure. Do not use a routine schedule as an emergency-treatment protocol.

Join the discussion

What do you measure before a change?

Sources and further reading

• Randy Holmes-Farley: water changes in reef aquaria