Last Updated: September 7, 2026
How to do a fish tank water change is mostly about preparation, since almost every problem afterwards traces back to water that went in untreated or several degrees off temperature. The routine below takes about fifteen minutes on a 20 gallon tank. Check the numbers first, then work through the steps in order.
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By David Nguyen
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Check the Numbers Before Starting
Two minutes of measuring prevents most of the trouble that follows a change. Knowing how to do a fish tank water change safely starts with the temperature and the volume, because those are the two figures that cause visible problems when they are wrong.
The numbers to have before you begin
Decide the volume first: 20 to 30 percent of the real water in the tank, which is roughly 15 percent below the label once substrate and hardscape are counted. A 20 gallon (76 L) tank holds nearer 17 gallons (64 L), so a 30 percent change moves about 5 gallons (19 L).
New water must land within 1 to 2 °F (about 1 °C) of the tank, so read the tank temperature on a separate thermometer rather than the heater dial. Dose dechlorinator for the volume of new water and confirm the label covers chloramine, not chlorine alone.
Test nitrate immediately before the change so the result reflects the peak the fish live in. Ammonia and nitrite should read zero in an established tank, and either of them appearing turns a routine change into a response rather than maintenance.
The gear and what each part does
The kit list is short and every item earns its place, which matters because a single missing piece is what turns a fifteen minute job into an improvised one. None of the kit listed here is expensive or particularly specialised.
A gravel vacuum sized to the tank does two jobs at once, removing water and lifting waste from the substrate before it decomposes. Too wide a tube on a nano tank empties it faster than you can control, so match the tube diameter to the tank volume.
A dedicated bucket, never used for cleaning products, and a thermometer complete the minimum kit. On tanks above 40 gallons (151 L) a faucet-connected changer removes the carrying entirely and is the difference between a schedule that holds and one that slips.
| Problem after a change | Reading or sign | Likely cause | What to do |
|---|---|---|---|
| Fish gasping within hours | Ammonia 0, nitrite 0 | Untreated chlorine or chloramine | Aerate, treat water, test again |
| Fish hiding, clamped fins | Temperature off by 4 °F (2 °C) | New water not matched | Stabilise slowly, match next time |
| Ammonia appears next day | 0.25 ppm or more | Filter cleaned in the same week | Change 25%, feed less, wait |
| Water cloudy afterwards | Readings normal | Substrate disturbed, fine debris | Leave it, clears in a day or two |
| Nitrate barely moved | Same as before change | Too small a percentage | Recalculate on real volume |
| Heater cracked or stopped | Temperature falling | Ran dry during the change | Unplug before draining next time |
When to do something other than a routine change
A tank that has gone months without maintenance needs several 15 to 20 percent changes on consecutive days rather than one large one, because pH, hardness and dissolved solids have all drifted and correcting them at once causes the shock people blame on the change itself.
If medication is in the tank, follow the product instructions rather than the normal schedule, since a change removes part of the dose. Activated carbon also strips medication from the water, so it comes out during treatment and goes back afterwards.
How to Do a Fish Tank Water Change Step by Step
The sequence matters considerably more than the speed. Knowing how to do a fish tank water change in this order keeps the heater safe, the substrate properly clean, and the fish well clear of any sudden swing in temperature or water chemistry.
Step one: prepare before you do a fish tank water change
Switch off and unplug the heater, and the filter too if the water line will fall below its intake. A glass heater exposed to air while running is the most common way one cracks, and a filter running dry can lose prime and stop moving water afterwards.
Prepare the replacement water while the heater cools. Mix hot and cold to land within 1 to 2 °F (about 1 °C) of the tank, add dechlorinator for that volume, and check the temperature with the same thermometer used on the tank rather than a different one.
Step two: how to do a fish tank water change at the substrate
Start the siphon and work across roughly a third of the substrate, pushing the tube in until the gravel lifts and settles rather than skimming the surface. Rotating which third you clean each week spreads the work and avoids stripping the whole bed at once.
Stop at the planned volume rather than at the point the bucket fills. Overshooting on a small tank is easy, and taking 60 percent when only 25 percent was planned turns a routine job into a chemistry swing that the fish then have to absorb.
Step three: refill, the stage most water changes fail at
Pour the new water gently against the glass or onto a plate rather than directly onto the substrate, which prevents a cloud of fine debris and keeps plants in place. Measure the gap behind the tank before buying a hose, since a kinked line is the usual reason refilling goes wrong here.
Plug the heater and filter back in once the water level covers both, then check the temperature again after twenty minutes. Feed lightly afterwards or skip the feeding entirely, since the fish gain nothing from a full portion immediately after that disturbance.
What Not to Do When Doing a Fish Tank Water Change
Three actions cause most of the losses that follow routine maintenance, and each of them has a specific and well understood mechanism behind it rather than being a general caution about handling a tank carefully during routine maintenance.
Do not change all the water at once
A full change strips buffering capacity and swings temperature, pH and dissolved solids simultaneously, which is far harder on fish than the cloudy water it was meant to fix. Keep single changes to 50 percent at most and space larger corrections across days.
Do not rinse filter media in the tap
Chlorine and chloramine kill nitrifying bacteria on contact, and the media holds nearly the whole colony. Rinse sponges and bio media in a bucket of the water just removed, squeeze gently, and replace only mechanical floss rather than the biological material.
Do not refill straight from the tap
Untreated water carries chlorine or chloramine directly into the filter, and boiling does not remove chloramine. Treat the new water in the bucket first, or dose the tank for the full volume before refilling when using a faucet-connected changer.
Making the Routine Repeatable
The reason to standardise how to do a fish tank water change is that a repeatable fifteen minute job survives a busy week, while an improvised hour does not. Most of that comes down to the equipment and to keeping a fixed order of operations every time.
The weekly routine
Change 20 to 30 percent every seven days, vacuum a rotating third of the substrate, and test nitrate before each one. Rinse mechanical media in removed tank water on a separate week from the substrate vacuum rather than doing both together.
Feed only what is eaten in one to two minutes across the week. Water changes remove the end product while feeding sets the production rate, so a routine that never changes will still lose ground if portions creep upward after new fish arrive.
What goes wrong over time
Equipment fails quietly rather than dramatically, and the failures show up as a change that went wrong rather than as an obviously broken tool. Checking the kit over occasionally costs far less than diagnosing the result days afterwards.
Siphons clog and lose flow, buckets pick up detergent residue from other jobs, and thermometers drift. The most consequential failure is a heater that has been unplugged and forgotten, which shows up as a temperature drop several hours after the change is finished.
Winter makes the temperature step harder, since tap water can arrive at 45 to 50 °F (7 to 10 °C) and needs more mixing to reach tank temperature. Where a utility switches to chloramine or issues a notice about the supply, check the dechlorinator label rather than assuming.
Early signs something went wrong
Fish gasping at the surface within hours points to untreated water or a temperature mismatch, and both call for immediate aeration and a check of the four readings. Hiding and clamped fins usually indicate a temperature swing rather than a toxin.
Ammonia appearing the following day suggests the colony was disturbed, most often by cleaning the filter in the same session. Change 25 percent, reduce feeding, and if unexplained behaviour continues once the water tests clean, consult an aquatic vet.
Frequently Asked Questions
These come up in the first month of maintaining a tank, usually after a change that seemed entirely routine at the time went on to produce a completely unexpected result in the tank the following morning.
How to do a fish tank water change without stressing the fish?
Match temperature within 1 to 2 °F (about 1 °C), treat the new water before it goes in, keep the change to 20 to 30 percent, and pour gently rather than directly onto the substrate. Nets and moving fish out are unnecessary and only add stress.
Do I need to remove the fish for a water change?
No. Fish stay in the tank throughout a normal change, since catching and moving them is far more stressful than the change itself. Leave enough water to cover them comfortably and work around them rather than chasing them about with the siphon tube.
Should the filter stay running?
Leave it running if the water line stays above the intake, and switch it off if it will not. A filter running dry can lose prime, overheat its impeller, and on some designs fail to restart at all, which costs far more than the few minutes saved.
How long does a fish tank water change take?
Around fifteen minutes on a 20 gallon (76 L) tank with a bucket and siphon, and longer on larger tanks unless a faucet-connected changer is used. If it regularly takes an hour, the equipment rather than the method is usually the thing to change.
See More:
Making It Routine
How to do a fish tank water change reduces to five things done in order: unplug the heater, prepare water at 1 to 2 °F (about 1 °C) and dechlorinated, siphon 20 to 30 percent while vacuuming a third of the substrate, refill gently, then restart the equipment.
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