Beginner maintenance guide
How to Do a Saltwater Aquarium Water Change
A water change is one of the simplest and most useful things you can do for a saltwater aquarium—but only if the replacement water is ready before you start. Here is the routine I use, what I check, and the tools that make the job easier.
Affiliate disclosure: This guide includes a few affiliate links. If you purchase through one, I may earn a commission at no additional cost to you. I only link to tools I use or that fit the job described.
My short answer
Prepare first, siphon second, refill slowly
For many beginner aquariums, a 10% to 20% weekly water change is a practical starting point. Mix the replacement saltwater completely, match its salinity and temperature to the aquarium, turn off anything that could run dry, remove the planned amount, refill slowly, and then restart and inspect every piece of equipment.
My water-change starting point
Python 2-Inch Gravel Vacuum
The Python gives me better control than a tiny siphon tube and removes water while pulling loose debris from targeted areas of the sand. Pinch the hose or lift the tube when fine sand starts climbing too high.
Watch the walkthrough
See the complete routine
This original walkthrough shows the physical process. The updated guide below adds the preparation, testing, equipment, and beginner-tank details that prevent the most common mistakes.
Why we do it
What a water change can—and cannot—do
Dilute dissolved waste
Removing old water lowers the concentration of nitrate, phosphate, and other dissolved material in proportion to the amount changed.
Restore a consistent baseline
A good salt mix replaces removed seawater with a known balance of major and minor constituents while restoring water volume.
Support maintenance—not replace it
A water change cannot fix overfeeding, poor source water, failed equipment, an overstocked tank, or waste that remains trapped in the aquarium.
How much water?
Use actual water volume, not the number on the box
Ten percent every week is a reasonable baseline. I often prefer closer to 20% when the aquarium, storage space, and maintenance routine make that practical. Larger corrective changes can be useful, but they demand more careful matching of temperature, salinity, and other parameters.
A “20-gallon” aquarium does not contain 20 gallons after rock, sand, equipment, and air space displace water. Calculate the real volume before dosing, mixing, or deciding how many gallons equal 10%.
Prepare the replacement water
Get these three jobs right before removing aquarium water
Mix the salt completely
I use Tropic Marin Pro Reef because it mixes cleanly and stores well in my mixing station. Follow the manufacturer’s directions for whichever salt you choose.
Match salinity
Measure the aquarium and replacement water with the same reliable tool. My default is a refractometer checked with calibration fluid.
Match temperature
Bring the new water close to the aquarium’s temperature before it enters the display. Use a correctly sized heater and pump in a safe mixing container.
Step by step
How I do a saltwater aquarium water change
1
Decide why you are changing water
A routine weekly change, a correction after a test result, and an emergency response are different jobs. Decide the goal and percentage before preparing the batch.
2
Test the replacement saltwater
Confirm salinity and temperature after the salt has dissolved. If you are making a large corrective change, also compare the parameters most likely to shift.
3
Turn off equipment that could run dry
Stop the return pump, heater, auto top-off, and any equipment whose intake or sensor will be exposed. Leave circulation running only when it will remain safely submerged.
4
Move debris into the water column
Use a turkey baster or small pump to blow detritus from rockwork and quiet corners before siphoning. Give suspended debris a route out of the aquarium.
5
Remove the planned amount
Mark the container or aquarium so you know when to stop. Watch livestock, heater placement, and pump chambers as the water level falls.
6
Clean the substrate selectively
Vacuum accessible dirty areas without trying to make the whole sand bed sterile. On bare-bottom tanks, siphon the piles of detritus you can see.
7
Add the replacement water slowly
Refill without blasting sand, coral, or fish. A small utility pump and hose make this dramatically easier than lifting and pouring heavy buckets.
8
Restart everything and inspect it
Restore the return pump, heater, circulation, auto top-off, skimmer, and other equipment. Confirm normal flow, temperature, water level, and operation before walking away.
Tools I actually use
Make the physical job easier
Python Gravel Vacuum
Use it to remove water and clean targeted sections of sand in the same pass.
Sicce Ultra Zero Utility Pump
I own three Ultra Zero pumps. They move water between my mixing station, RO reservoirs, wheeled water-change container, and aquarium so I do not have to lug buckets.
Python No Spill Extension
Add only the hose length you need when the aquarium, mixing container, and drain are far apart. Test and tighten every connection before moving water.
Python No Spill Clean and Fill Hook
The hook secures the fill hose over the aquarium rim so it does not kink or slip out. It frees both hands, but I still never leave a filling aquarium unattended.
BRUTE Trash Can
Use a sturdy, clean container reserved for aquarium water. Choose the capacity that matches the batch you can safely store and move.
BRUTE Trash Can Dolly
A full water-change container is extremely heavy. The matching dolly lets me roll it into position instead of trying to carry it.
Avoid these mistakes
The problems I see most often
- Removing aquarium water before the replacement batch is completely ready.
- Matching salinity but ignoring temperature—or matching temperature while guessing at salinity.
- Changing a percentage based on the tank’s label instead of its actual water volume.
- Letting a heater, pump, or auto top-off sensor operate while exposed to air.
- Stirring the entire sand bed aggressively and releasing more waste than the siphon removes.
- Pouring new water directly onto sand, coral, or a resting fish.
- Forgetting to restart one piece of equipment after the job is finished.
My rule: do not walk away until you can see normal flow, confirm the heater and auto top-off state, and verify that the water level is correct.
Water-change FAQ
Common beginner questions
How often should I change saltwater aquarium water?
Weekly is an excellent routine for many beginner aquariums. The percentage depends on the tank’s actual water volume, feeding, stocking, test results, and how consistently the rest of the filtration is maintained.
Can I use tap water?
I recommend purified RO/DI water for mixing saltwater and replacing evaporation. Untreated tap water varies by location and can introduce substances you do not want in a reef aquarium.
Should I clean all the sand at once?
No. Clean the dirty, accessible areas while leaving established surfaces and livestock habitat intact. Deep or neglected sand beds require extra care.
Do I need to turn off my protein skimmer?
Yes if the water level will fall below its operating range or the skimmer will overflow when the return pump stops. Restart it only after the system’s water level is stable.
Continue from here
Build water changes into a repeatable maintenance routine
Weekly Aquarium Maintenance
See the rest of the jobs I pair with a water change so nothing important gets forgotten.
Choose a Salt Mix
Compare the mixes I trust, including the one I use in all of my tanks.
Build a Testing Setup
Use the right tools to verify salinity and the parameters that matter for your aquarium.
Updated substantially August 2026. This hybrid revision preserves Matthew’s original water-change explanations and video while removing duplicated legacy sections and aligning the routine, internal links, and equipment recommendations with the current My First Fish Tank guide system.
Reference notes: The procedure also reflects general aquarium husbandry guidance from the Merck Veterinary Manual. Always follow the mixing and storage directions provided by the manufacturer of your chosen salt.