The short answer
Ready the new water first: treat it for chlorine and chloramine and match the tank’s temperature. Then unplug the heater and filter, siphon out part of the water while vacuuming the gravel, refill slowly and plug back in once full. Makers’ amounts differ: Aqueon caps a change at 25%, API at 30%.
Steps are from freshwater sources, for an established freshwater tank; a cycling tank starts with our guide on how to cycle a fish tank. Glass, decor and filters: see our fish tank cleaning guide.
What you need
Cleaning tools and supplies:
- A bucket kept for the tank. Aqueon, an aquarium equipment maker: “Don’t use buckets, scrub pads or any other equipment on your aquarium that have been used with soap or other chemicals.”
- A siphon (gravel vacuum) or a faucet water changer.
- A conditioner “that treats chlorine AND chloramine”, as Dr. Jessie Sanders, an aquatic veterinarian, puts it.
- A thermometer and a test kit for ammonia, nitrite and nitrate.
The two conditioners below are compared in our water conditioner roundup, the master kit in our freshwater test kit roundup.
| # | Preview | Product | Price | Link |
|---|---|---|---|---|
| 1 |  | Seachem Prime -500 ml/ 17 fl. oz. | Chemical Remover and…Concentrated ammonia binder | $16.62 Prime | Check on Amazon |
| 2 |  | API Stress Coat Aquarium Water Conditioner 16-Ounce BottleAloe slime-coat formula | $10.98 Prime | Check on Amazon |
| 3 |  | API Freshwater Master Test Kit 800-Test Freshwater Aquarium…Liquid master kit | Check on Amazon |
How much water to change, and how often
Each source gives its own water change percentage and frequency, all partial. Aqueon’s two pages differ.
| Source (type) | How much | How often |
|---|---|---|
| Aqueon (maker), do’s and don’ts | 10% or 25%; avoid more than 25% | 10% weekly or 25% every two weeks |
| Aqueon (maker), cleaning checklist | At least 25% | During a full clean |
| API (maker), FAQ | 25% | Every two weeks |
| API (maker), routine care | 10-30%; not over 30% | Test weekly-biweekly; change when levels are up |
| GloFish (maker), sheet | About 33% (its web page: 30%) | New tanks: at 2 and 4 weeks, then monthly |
| Python (maker), sheet | No more than one third (also its boxed note) | Very dirty tank: wait a week to repeat |
| Dr. Sanders (aquatic vet) | Start at 10% or 25%; never over 50% | 10% weekly or 25% every other week |
| Practical Fishkeeping (magazine) | About 25% | Once a week |
Aqueon: “Avoid water changes of more than 25% as larger water changes can stress your fish and upset the balance in your aquarium.” API, on the beneficial bacteria: “It is not recommended to do more than 30% because it may cause loss of bacteria and be stressful on the fish.” Dr. Sanders: “Do not remove more than 50% of the tank water at a time to avoid sudden swings in temperature and pH.” GloFish’s schedule also covers a tank getting “a substantial number of new fish”.
A 2009 Practical Fishkeeping answer (“it really depends on your tank”) suggests comparing the tank’s nitrate with your tap water’s: “If you’re doing enough water changes the two should be about the same.” By its rule, “A difference of more than 40ppm means you need to change more water more often.”
How to change the water, step by step
Test first
API says to “Test the aquarium water weekly-biweekly” and change 10-30% when levels are elevated.
Ready the new water: temperature and conditioner
Aqueon: “Replacement water should be the same temperature as the aquarium water” and “Always treat tap water with a water conditioner to neutralize chlorine and ammonia before adding it to the aquarium.” Aquarium Co-Op, an aquarium retailer, says chloramine “is often used in tap water, and it does not evaporate over time.” GloFish lets treated water “sit next to your tank for several hours to help ensure that it is about at room temperature. This is very important because a quick temperature change can be fatal to your GloFish.” Our reading for a heated tank: match the tank, as Aqueon says, using the thermometer. Dose by the label.
Unplug the heater and filter
Aqueon’s checklist opens with “Unplug the filter and heater.” Fluval’s AquaClear filter manual: “Always unplug or disconnect all appliances in the aquarium from electricity supply before placing hands in water, before putting on or taking off parts and while the equipment is being installed, maintained or handled.” Hydor, a heater maker, gives two lead times. Its Theo manual: “To avoid possible breakages due to thermal shock, the heater should be disconnected from the power supply at least 5 minutes before taking it out of the water or before adding or changing the water.” Its support page: “When undertaking water changes Hydor recommend that the heater is turned off and unplugged from the power socket at least 30 minutes before any waterchange.” Follow your heater’s manual and wait that long before siphoning.
Siphon out water, cleaning the gravel
Co-Op notes that “Aquarium siphons use gravity”, so the bucket goes below the tank. With the hose in the bucket and the tube filled underwater: “Raise the tube out of the water and above the aquarium rim until water starts flowing through the hose and into the bucket”, then lower it back. Dr. Sanders: “Press the end of the siphon into the substrate, allow the gravel or sand to move 1/4 to 1/3 of the way up the tube, then pull the siphon up, breaking the siphon.”
Refill slowly
GloFish: “Slowly add the new water into the aquarium.” Dr. Sanders pours slowly or uses a smaller container “to avoid a heavy current in your tank that may hurt your fish.”
Plug in only when the tank is full
Hydor: “The Heater can ONLY then be turned back on when the Aquarium is full and the heater is completely submerged.” Its manual: “Do not connect the heater to the power supply when it is not submerged in water.” Fluval: “Do not allow filter pump to run dry.”
With a Python faucet changer, steps 2, 3 and 6 still apply: you still treat the water, and set its temperature at the faucet. When draining, if fish or gravel are drawn into the tube, Python says “you may interrupt the cleaning process by temporarily closing the switch and letting the object drop back to the gravel bed”, and cautions: “CAUTION if your tank has sand, do not force switch to close as this will either score it or break it.” To fill, in its order: take the gravel tube out and close the switch; set the faucet to the tank’s temperature; turn the pump to its FILL position; then hold the tube over the tank and “slowly turn the switch C to the OPEN position”. Its caution: “DO NOT LEAVE THE SWITCH IN THE CLOSED POSITION FOR MORE THAN THE TIME IT TAKES FOR YOU TO GET TO YOUR AQUARIUM.” Python says the conditioner can go “into the stream of water as you are refilling the tank”; the other placements are below. Follow Python’s sheet for your model.
Where the conditioner goes
Sources differ, even API’s two pages:
- In the new water. API’s FAQ: “the new water should be treated with a dechlorinator like API STRESS COAT™ water conditioner before being added to your tank.” Aqueon’s checklist: “add water conditioner to new water and then refill the aquarium”.
- In the tank, for the whole volume. API’s routine-care page: “It is safe to dose for the entire volume of the aquarium even during partial water exchanges.” Seachem, a water-care maker, allows Prime in the tank, dosed by tank volume, but prefers the new water.
- Before a hose refill. Practical Fishkeeping’s Jeremy Gay, on small daily changes through a Python siphon: “But remember to put the right amount of tapwater conditioner into the tank before you refill, so that you’re not exposing your mature filter to chlorinated tapwater.”
Mistakes and troubleshooting
- Changing the filter cartridge the same day. API’s routine-care page changes it “every 4-6 weeks on different days from doing a water exchange.” Aqueon’s checklist, Dr. Sanders and our cleaning guide clean filter media in the same session, so check your filter’s manual.
- Topping up instead. GloFish: “It is not enough to replace water that has evaporated.”
- Ammonia or nitrite above zero after the change. See how to lower ammonia or nitrite.
- Nitrate keeps coming back. See Practical Fishkeeping’s tank-versus-tap comparison above, and how to lower nitrates.
- Fish look unwell. Test the water and contact an aquatic veterinarian.
Other maintenance tasks are in our Tank Setup & Care section.
Ready to buy?
Our water conditioner roundup compares eight conditioners on their makers’ claims.
Frequently Asked Questions
Q. Should you take fish out when changing water?
No. GloFish: “Do NOT remove the fish from the aquarium when making a partial water change.”
Q. Should you ever do a 100% water change?
API: “No, it is not recommended to do that because it will cause the aquarium to go through the nitrogen cycle and potentially causing great stress or death of your fish.”
Q. Do you need water changes with a filter?
Yes, says API: “Performing partial water exchanges will dilute the fish waste and organic debris accumulating in the water.”
Q. How do you change water without a siphon?
API describes “scooping out the top layer of your aquarium water and replacing it with (conditioned) tap water.”
Q. How long can a tank go without a water change?
None of these sources gives a maximum. Practical Fishkeeping suggests comparing your tank’s nitrate with your tap water’s; more than 40 ppm apart means more water, more often.
Sources (17)
- Aqueon, Do’s and Don’ts
- Aqueon, Cleaning Checklist
- API, FAQs
- API, Routine Aquarium Care
- GloFish, water change sheet
- Python, instructions
- Fluval, AquaClear manual
- Hydor, Theo manual
- Hydor, support page
- Seachem, Prime
- Dr. Sanders (vet), cleaning
- Dr. Sanders (vet), water changes
- Dr. Sanders (vet), resolutions
- Practical Fishkeeping (magazine), how often
- Practical Fishkeeping, easier changes
- Aquarium Co-Op (retailer), gravel vacuum
- Aquarium Co-Op (retailer), cleaning
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