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Maintenance mechanics

Aquarium water changes, done with judgment

Prepare suitable replacement water, remove part of the old water, then refill and check the equipment. The right amount depends on your tank's test results and the needs of its animals.

2 min readUpdated September 27, 2026By FishicianPublished August 24, 2026How we source

The short answer

  • Prepare before you drain. Treat chlorine or chloramine as needed, match temperature and, for marine tanks, match salinity.
  • Choose the amount from the tank's needs. Use water-test trends and a repeatable routine; there is no percentage that fits every aquarium.
  • Keep the biofilter established. Protect biological media from drying, untreated chlorinated water and wholesale replacement.
  • Topping off isn't a water change. Evaporation leaves salts and minerals behind. Adding water back does not remove accumulated waste.
A hand using a gravel siphon in an aquarium, with its clear hose draining into a lower bucket
A gravel siphon carries water into a bucket below the tank. Keep the outlet secure and watch the falling water level.
Water change checklist
Before
  • Check tank and source water
  • Prepare and condition new water
  • Measure temperature; marine salinity too
During
  • Power down heaters and pumps that could run dry
  • Siphon waste where it suits the tank
  • Refill gently
After
  • Check equipment restarted
  • Watch every fish
  • Log volume, treatment and readings

1. Prepare the replacement water

Use dedicated aquarium buckets and hoses. Check your source water so you know what needs treating.

  • Tap water: use a conditioner rated for the disinfectant in your supply. Follow its current label, including whether the dose is based on new water or the whole tank. Do not rely on standing water to remove chloramine.
  • Marine water: dissolve aquarium salt in a separate container according to the salt manufacturer's instructions. Measure salinity before adding it.
  • Both: measure temperature and match the tank. Replacement water can restore buffering only if it contains suitable alkalinity; unmineralized RO/DI water does not replenish it.

2. Remove water and accessible waste

Follow the equipment's shutdown instructions before lowering the water. Heaters and pumps that could be exposed must not run dry.

Siphon the planned amount, watching the intake and fish throughout. Remove accessible debris without uprooting plants or digging through a planted substrate. A water change does not require replacing the filter media; see safe filter maintenance.

3. Refill, restart and observe

Add the prepared water gently. Restore the operating water level, then restart equipment as directed. Confirm filter flow, heating and aeration rather than assuming everything restarted.

If fish gasp or deteriorate: support aeration and promptly check temperature, water quality, conditioner use and equipment. The timing is a clue, not a diagnosis. Seek urgent aquatic-veterinary help for severe or unexplained distress.

How much, and how often?

Stocking, feeding, plants, filtration and source water all affect the routine. Log the volume changed and compare water-test trends between changes. Adjust the amount or interval to keep conditions suitable for your animals.

A neglected tank needs care: when its pH and chemistry differ greatly from the replacement water, a large change can cause shock or increase ammonia toxicity. Seek help planning staged correction. Acute contamination may instead need prompt dilution with suitable water; do not delay an emergency response to follow a routine schedule.

Sources

Each source is listed with the part of this guide it supports.