10 to 25 percent
Most tanks do well with a 10 to 25 percent change every week or every other week.
Routine care
Gravel Vacuuming and Water Changes
Partial water changes remove waste you cannot see and keep the tank steady. Here is how much to change, how to run a siphon, and how to refill without shocking your fish.
How much, how often
Small and regular wins
Waste turns into nitrate that most filters do not remove. Water changes take it out and put back minerals the fish use.
Watch nitrate
If nitrate keeps climbing between changes, change water more often or a little more each time.
Busy tanks need more
Crowded tanks and messy eaters such as goldfish or large cichlids usually need bigger or more frequent changes.
Catch up slowly
If it has been months, do several smaller changes over a week or two. One huge change can shift the water too fast.
What you need
Get it ready first
Keep these tools for the aquarium only. Soap and cleaning residue from household buckets can harm fish.
Siphon and bucket
A gravel vacuum is a wide tube on a siphon hose. Pick one sized to your tank.
- Aquarium-only bucket or a hose to a drain
- Towel for drips and spills
- Fine mesh for the intake in shrimp or fry tanks
Water conditioner
Tap water often contains chlorine or chloramine. Both harm fish and filter bacteria.
- Use a conditioner that treats chloramine
- Dose for the amount of new water, per the label
- Refilling straight from a hose? Follow the label's directions for that
Thermometer
Match new water to the tank within a degree or two. Do not guess by hand.
- Check the tank first, then the new water
- Mix hot and cold tap water in the bucket
- Adjust and recheck until the reading matches
Pre-mixed saltwater
Mix new saltwater ahead of time in a clean container with a small pump and heater.
- Use RO/DI water and follow the salt mix label for mixing time
- Match salinity and temperature to the tank before you start
- Check salinity with a refractometer or hydrometer
Siphon technique
Do these in order
Have the new water ready before you remove anything. Then the tank is only low for a few minutes.
Power down
Turn off equipment that should not run in low water.
- Unplug the heater before the water drops below it
- Turn off filters whose intake will be above the water
- Dry your hands before touching any plug
Start the siphon
Keep the bucket lower than the tank. Start the flow with the bulb or by filling the tube underwater.
- Do not start it with your mouth
- Keep the hose end in the bucket
- Pinch the hose to slow the flow
Vacuum in sections
Push the tube into the gravel, let the dirt lift, then move to the next spot before gravel climbs the tube.
- Clean part of the bottom each time, not every inch
- Get open areas and the front glass where waste collects
- Stop when you reach the amount you planned
Refill gently
Add conditioned, temperature-matched water slowly so you do not blast the substrate.
- Pour onto a plate, rock, or your hand
- Turn the heater back on once it is fully underwater
- Restart filters and check they are flowing
Different bottoms
Planted tanks, sand, and reefs
Not every bottom should be dug into. Match the technique to what is in your tank.
Skim, do not dig
Hover over the surface and clean open areas. Digging into roots or nutrient soil clouds the water and disturbs plants.
Hover above it
Hold the tube about an inch above the sand. Debris lifts while the heavier sand falls back down.
Top layer only
Siphon only the surface of reef sand. Digging deep into an old sand bed can release trapped waste.
Not a water change
Evaporation leaves minerals and salt behind. Top off saltwater tanks with fresh RO/DI water, never saltwater.
Avoid these
Common mistakes
Water changes are safe when the new water matches the old. Most problems come from skipping that step.
Skipping conditioner
Even a small amount of untreated tap water can stress fish and harm filter bacteria.
Guessing temperature
Water that feels fine to your hand can be several degrees off. Use a thermometer.
Cleaning everything at once
A deep gravel clean and a filter clean on the same day removes a lot of bacteria. Space them out.
Unmatched saltwater
Fresh-mixed or unmatched saltwater can swing salinity and pH. Mix ahead and check it first.