How to do a water change, step by step

Every other piece of goldfish care builds on this one. Filters convert ammonia into nitrate, but in most goldfish tanks it is the water change that takes the nitrate out, and nothing else dilutes the dissolved organics and mineral drift that build up quietly between changes. This guide is about how to do it well. For how often and how much for your particular tank, see how often to change goldfish water; the short version is 25 to 50 percent weekly, guided by nitrate readings.

The equipment

A siphon with a gravel vacuum head: a wide tube that lifts debris out of the substrate as the water flows through it. Manual, squeeze-bulb and self-priming versions all work. Width matters more than length on a goldfish tank, and on a bare bottom you need no head at all — the siphon buying guide covers choosing one and when a tap-connected system earns its place.

One or two buckets used for nothing else. A bucket that has ever held soap, bleach or garden chemicals is not an aquarium bucket. Label them.

Water conditioner, also sold as dechlorinator. Check that the label says it treats both chlorine and chloramine; almost all modern products do.

A thermometer for the new water. Digital or glass, it does not matter, but you need to read the bucket as well as the tank.

Optional, and worth it for larger tanks: a tap-connected hose system, a jug or a length of hose for refilling gently, and a test kit to record readings before you start.

A water change, step by step

1. 1 Test first Nitrate tells you how much to change. Ammonia or nitrite above zero means change more, sooner.
2. 2 Siphon out 25⁠–⁠50% Vacuum the substrate as you go. Old water goes to the drain, never back in.
3. 3 Treat the new water Dechlorinator for the full volume you are adding. Use a conditioner labelled for chloramine; test for ammonia afterwards.
4. 4 Match the temperature Within a degree or two of the tank. A sudden temperature change stresses the fish.
5. 5 Refill slowly Pour onto a plate or decor so the substrate is not stirred and the fish are not blasted.

The steps

1. Switch off the heater, if there is one, before the water level drops below it, and switch off the filter if lowering the level would leave its intake in air. Lights on or off as you prefer; some fish are calmer in dimmer light.
2. Start the siphon into the bucket and work the gravel vacuum across the substrate in sections, pushing it into gravel or hovering above sand. Stop when you have removed your target amount, typically a quarter to a half of the volume. Marking the glass at the 25 and 50 percent lines with a strip of tape saves guesswork.
3. Fill the bucket with tap water and adjust it until the thermometer reads within a degree or two of the tank.
4. Add conditioner to the bucket, dosed by the label for the amount of water in it, and stir. If you refill from a hose straight into the tank, dose the tank for its full volume before the water goes in.
5. Pour the new water in slowly, onto a plate, a rock or your hand, so it does not blast the substrate or the fish. A hose held low in the tank with the flow turned down does the same job.
6. Restart the heater and filter, check that the filter primes and flows, and watch the fish. They should be calm and curious within a few minutes.

The tank cleaning guide covers what else to do while the water is low, from scraping glass to rinsing filter media.

Starting the siphon

A step that stops many beginners, and it takes one attempt to learn.

The thumb method, which always works. Submerge the whole tube and the vacuum head so the entire thing fills with water and no air is trapped. Put a thumb firmly over the far end, lift that end out of the tank, hold it lower than the tank’s water level, and let go over the bucket. Gravity does the rest.

The only rule that matters: a siphon runs because the outlet is lower than the water in the tank. If the bucket is on a chair, nothing will happen. On the floor, it will.

Squeeze bulbs and self-priming heads do the same job without the thumb: hold the head in the water and pump a few times until flow starts. They are worth having and they clog with sand, which is fixable by rinsing.

Do not start a siphon by mouth. It works and there is no reason to do it, since the thumb method is as fast and tank water can carry bacteria that make people ill: the CDC names Salmonella and mycobacteriosis (fish tank granuloma).

And do not leave a running siphon unattended. A bucket overflows faster than seems possible, and the tank keeps draining until the level reaches the tube. Marking the glass at the 25 and 50 percent lines with a strip of tape means you can see the target without watching the bucket.

Vacuuming gravel, sand and bare glass

Three different techniques, and using the wrong one is a common frustration with the whole job.

Gravel: push in. Drive the vacuum head down into the stones so the water churns through them. Debris lifts up the tube and the gravel, being heavier, falls back. Work in sections — a third of the bed at each change is plenty, which spreads the disturbance and does not strip the bacteria living in there all at once.

Sand: hover. Sand is light enough to go straight up the tube, so hold the head a centimetre or two above the surface and let the flow lift the waste off the top. Sand’s advantage is that debris sits on it rather than sinking in, so hovering collects almost everything. If sand does start rising, pinch the hose to slow the flow or lift the head a little — the debris will still go and the sand will drop back.

Bare bottom: just hover, and everything is visible. This is why breeders use it, and the substrate guide covers the trade-off against a goldfish’s foraging.

Under the decor is where it actually collects, in every case. Lift or move ornaments occasionally rather than vacuuming around them forever.

Matching the temperature

Goldfish cope well with gradual temperature change and badly with sudden change. Refilling with water several degrees colder than the tank is a classic trigger for stress, and keepers often report an ich outbreak after a sharp chill. Aim to have the new water within about 1 to 2 °C (2 to 4 °F) of the tank, and remember that a small tank is affected far more by a bucket of mismatched water than a large one. Mixing hot and cold at the tap is the usual method; if your home has an old hot-water system, warming a bucket of cold water with a kettle avoids any metals the hot pipes might carry. The water temperature guide explains what stability does for goldfish. Matching the temperature also stops a second problem: cold water warming in the tank gives up dissolved air, and a large cold change can leave the glass, and sometimes the fish, covered in fine bubbles.

Chlorine and chloramine

Tap water is disinfected with chlorine, chloramine or both, and both damage fish gills. Chlorine is a gas that slowly leaves standing water; chloramine is stable and does not. A conditioner neutralizes both in seconds, which is why “let the water stand overnight” is advice from another era.

Dose by the label. Conditioner is one of the few aquarium products where a modest overdose is harmless, so round up rather than down. Tap water, dechlorinator and source water goes further into what is in your supply — the softener that ruins a tank, nitrate you cannot dilute away, and why well water has to be aerated before you test it.

How much to change

A quarter to a half of the tank each week suits most goldfish setups, with heavier stocking at the higher end. Bigger changes are not dangerous in themselves: what stresses fish is a sudden difference between old and new water, not the volume. If the new water is at the same temperature, treated, and from the same tap the tank has always been filled from, a 50 percent change is as gentle as a 25 percent one.

The exception is a tank that has been left too long. If nitrate has climbed far above 40 mg/L, or the pH has drifted well away from the tap’s, do a series of 25 percent changes a day or two apart rather than one huge one, so the fish adjust in steps. If you are not sure of your tank’s real volume, the aquarium volume calculator will tell you, and it matters for dosing conditioner.

The twenty-minute version

Once the equipment lives in one place and the routine is habit, the whole job is short. This is what it looks like when it is working:

1. Test nitrate while the kettle of warm water is running. One minute.
2. Heater off, filter off if its intake would be exposed. Ten seconds.
3. Siphon to the tape mark, vacuuming a third of the substrate. Five to ten minutes.
4. Fill and temperature-match the bucket, or set the tap on a hose system. Three minutes.
5. Conditioner, dosed and stirred. Ten seconds.
6. Refill gently. Three minutes.
7. Everything back on, and a proper look at the whole tank — not just the fish — before you walk away. One minute.
8. Write the nitrate reading down. Ten seconds, and it is the step that turns a chore into a record.

The parts that make it twenty minutes rather than forty are the tape mark on the glass, the equipment stored together, and not carrying buckets.

While the water is down

The level being low is an opportunity, and taking it is what keeps the occasional jobs from ever becoming jobs.

Easier at half level than at full:

• Scrape the front glass, and wipe the inside rim where a film builds.
• Rinse the mechanical sponge in the bucket of tank water you have just siphoned out, not under the tap. Chlorine and chloramine can damage the bacteria in it, and there is nothing to gain by risking that; the filtration guide covers media.
• Check the heater, its suction cups and its guard, and unplug it before the level drops past it.
• Look behind and under the decor, for trapped food, for a dead snail, or for a fish you have not consciously seen in a fortnight.
• Trim or rearrange plants, which is far easier with your arm in less water.
• Check the filter intake and the pre-filter sponge.
• Wipe the outside glass and the lid, which nobody does and which is most of the difference between a tank that looks cared for and one that does not.

None of these needs doing weekly. Doing one or two at every change means the list never accumulates, and the tank cleaning guide covers the deeper versions.

When it goes wrong

Short list, because the same handful of things happen to everyone.

• The siphon will not start. The bucket is too high, or air is trapped in the tube. Lower the bucket; resubmerge the whole tube.
• It stops mid-flow. The head has clogged, usually with a leaf or a large piece of debris, or the level has dropped to the intake. Lift, clear, restart.
• Sand is going up the tube. Raise the head or pinch the hose — see above.
• A fish is investigating the vacuum. Move the head away rather than the fish. Goldfish are curious about the siphon and quickly learn it stirs up food; the risk is to long fins and to protruding eyes, so keep the intake away from a telescope or a bubble eye rather than trying to shoo it off.
• The bucket overflowed. Mark the tank, not the bucket, and stop watching the wrong container.
• A fish went up the tube. Almost only possible with fry or a very small fish, and the answer is a piece of sponge over the intake or a smaller-bore siphon. A fish that has been through is usually unharmed if it lands in water.
• You forgot the conditioner. Dose the tank immediately for its full volume, add aeration, and test ammonia over the next day or two — red streaks and gasping at the surface cover what to watch for.
• You forgot to switch the heater back on, which is easily done, and is why the last step of the routine is a look at the whole tank rather than at the fish.

Doing it without stressing the fish

Goldfish learn routines. Do the change at the same time each week, move slowly, and never chase a fish away from the siphon; it will move. Keep the intake of the vacuum away from long fins and telescope eyes. Refill gently, and do not top the tank up and then immediately feed. Most goldfish come to treat the siphon as a curiosity, and a good number will follow it around hoping for stirred-up food.

Hose systems for larger tanks

Above about 150 litres (40 US gallons), buckets become a chore, and a tap-connected system, sold under the Python name and several similar ones, pays for itself quickly. A venturi at the tap creates suction to drain the tank through a long hose, then a valve reverses the flow to refill straight from the tap. Set the tap to the tank’s temperature with a thermometer under the running water, dose the whole tank with conditioner before the refill starts, and keep the flow gentle at the tank end. The water is treated in the tank as it arrives, which is standard practice with these systems and works well.

Water changes on a pond

A different job, and the weekly aquarium percentage does not transfer.

A pond usually needs far less, because the volume is large and the planting does real work. Many pond keepers change a modest proportion occasionally rather than a set fraction weekly, and let the test readings decide — goldfish ponds covers the reasoning.

The mechanics are different too:

• A pump rather than a siphon for anything beyond a few buckets, with the outflow going somewhere that will not flood.
• Conditioner at hose volumes is the awkward part. Dose for the volume being added as the hose runs, or add in smaller amounts so the dilution into a large pond does the work.
• Do not strip the bottom. The layer of detritus in a pond hosts a great deal of the biological processing, and a thorough clean-out can set a pond back a season. Remove leaves and gross debris; leave the rest.
• Season matters. Large changes are for the growing season — a pond through the year covers when to do what, and winter is not the time.

When the tap water itself is the problem

Occasionally the water you are adding is part of the difficulty. Test your tap water once, straight from the tap and again after it has stood for 24 hours, because dissolved gases can shift the pH reading. In some areas tap water carries nitrate at 20 mg/L or more, which sets a floor no water change can go below; in others the carbonate hardness is so low that pH drifts between changes. The water testing guide explains what each reading means. Fixes range from more plants and lighter stocking to nitrate-removing resins or blending in reverse-osmosis water, and which is sensible depends on how far off your water is.