How Often Should You Change Fish Tank Water?

This page covers freshwater tanks; conditioners are compared in our water conditioner roundup.

What Each Source Says

Each source gives its own water change percentage and frequency for routine care; Aqueon’s two pages differ.

Source (type)	How much	How often, or when
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, routine care	10-30%; not over 30%	Test weekly-biweekly; change when levels are up
Fluval (maker), care guide	About 10-20%; never more than 40%	Every 1-2 weeks, depending on aquarium conditions
Tetra (maker), nitrate kit	No routine figure; around 30% for excess nitrate (kit FAQ)	Test weekly; regular changes if nitrate is high
GloFish (maker), sheet	About 33% (its web page: 30%)	New tanks: at 2 and 4 weeks, then monthly
Dr. Sanders (aquatic vet)	Start at 10% or 25% for tanks; not over 50%	10% weekly or 25% every other week
Practical Fishkeeping (magazine, 2009)	About 25%, “for most people”	Once a week; “it really depends on your tank”
TFH Magazine (magazine, 2011 issue)	50% or more “much more effective” (routine care)	Weekly at minimum; “more is often better”
Aquarium Co-Op (retailer)	No figure	“it totally depends!”

After an ammonia reading

A different question: how much to change once ammonia shows. Aqueon’s 25 to 50% here goes past its own 25% routine limit.

Source (type)	How much	When
Aqueon, ammonia guide	25 to 50%	Any ammonia reading
UF/IFAS Extension (university)	25% to 50%	Ammonia present, if the new water has none

Where they differ

• How often. Weekly (Practical Fishkeeping, a UK fishkeeping magazine, in 2009; TFH at minimum), every two weeks (API’s FAQ), every 1-2 weeks (Fluval), or monthly after the first four weeks (the new-tank schedule of GloFish, a fish and aquarium-kit maker). Aqueon and Dr. Jessie Sanders, an aquatic veterinarian, offer 10% weekly or 25% every two weeks. Co-Op, an aquarium retailer: “Some people say once a week, while others say once a month.”
• How big one change can be. A real disagreement. 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.” Fluval: “Never change more than 40% of the water at any one time, and never empty the aquarium completely to clean it. Doing so will stress your fish and disrupt the natural balance of the tank.” Dr. Sanders: “Do not remove more than 50% of the tank water at a time to avoid sudden swings in temperature and pH.” TFH Magazine, a fishkeeping magazine, in a 2011 issue, on routine care: “Large changes of 50 percent or more are much more effective at keeping wastes sufficiently diluted.” For a tank with very high nitrate, TFH itself says to “start slowly. Lightly vacuum the substrate until no more than 15 percent of the water has been removed.” For a tank kept up with routine changes, its 50 percent or more is above the Aqueon, API and Fluval limits, and above Dr. Sanders’s for anything over 50%.

What Changes the Answer for Your Tank

• Fish, food and tank size. Practical Fishkeeping’s 2009 answer says nitrate “is produced constantly and the rate at which levels rise will depend on the tank volume, number and size of fish and what and how much they are fed.” Fluval says the frequency “will depend on your filter type and aquarium setup”.
• Live plants. The makers stress different things. Tetra’s nitrate kit FAQ says live plants, with its own FilterActive and EasyBalance products, reduce the need for frequent water changes. Seachem, a water-care maker: “plants should not be used as a replacement for water changes or good aquarium maintenance.” Neither gives a schedule for a planted tank.
• A new tank that is still cycling. Aqueon’s nitrogen cycle page ties changes to ammonia and nitrite readings: “Until the nitrogen cycle is fully established, test the ammonia and nitrite levels every two to three days. You’ll likely need to change out about 10 to 25% of the water every few days to ensure ammonia and nitrites don’t reach toxic levels.” Its first-60-days page, for days 15 to 30 after setup: “There are different philosophies on how much and how often to change water, but 10% to 25% every 1 to 2 weeks is a good rule of thumb.” and “Don’t vacuum the gravel yet, as you may disrupt the good bacteria that are just starting to colonize your aquarium.” Fluval, for the early phases, differs on gravel: “A 10% change per week is recommended, using a Gravel Cleaner alongside Fluval Water Conditioner, Fluval Biological Enhancer, and Fluval Biological Cleaner each time.” GloFish’s schedule (table) also covers “a substantial number of new fish”. For the cloudy bacterial bloom after the first fish, Aqueon only: “Resist the desire to do a water change! Water changes, especially large ones will only prolong the bloom and may stress your fish.” Its day-5-to-15 step still says to test ammonia and nitrite, and: “Use Aqueon Ammonia Neutralizer, water changes or chemical filtration media to prevent them from reaching dangerous levels.” See our guide to cycling a fish tank.

Let Your Test Kit Set the Schedule

API Freshwater Master Test Kit (step 1), from our freshwater test kit roundup: “Freshwater only”; “No GH, KH, chlorine or phosphate test, so hardness needs a second kit”. Seachem Prime (step 4), from our water conditioner roundup: Seachem says Prime “removes chlorine, chloramine and detoxifies ammonia, nitrite, and nitrate”; its FAQ says Prime “does not remove ammonia, nitrite, or nitrate from the system” but binds them “for up to 48 hours”, and the roundup “found no independent test” of the binding claim. “A binder can change your ammonia reading”: Seachem’s Prime FAQ says “a Nessler based kit will not read ammonia properly if you are using Prime”; with a salicylate kit, the roundup says, “read right away”.

What Not to Do

• Do not empty the tank. Aqueon says beneficial bacteria “live in places like a biofilter, gravel bed, or on solid objects in the aquarium”. API, asked about a 100% change: “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.”
• Do not go over the per-change limit your own source gives for routine changes (25%, 30%, 40% or 50% in the table; TFH differs).
• Do not top up instead. GloFish: “It is not enough to replace water that has evaporated.”
• Do not use untreated or wrong-temperature water, or plug the heater in early (step 4).

Your equipment’s manuals and your conditioner’s label come last. More maintenance tasks are in our fish tank cleaning guide, our guide to cleaning aquarium glass and our Tank Setup & Care section.

Frequently Asked Questions

None of these sources gives a maximum. Practical Fishkeeping’s 2009 tank-versus-tap nitrate method is in step 3.

Yes. API, on a large filter: “Fish waste and organic debris still accumulate in the water over time.”

Aqueon: “Change filter cartridges at least once a month. Check them weekly and rinse as needed.” API changes it “every 4-6 weeks on different days from doing a water exchange”; the different-day part is API’s alone, as Aqueon’s checklist and Dr. Sanders clean filter media in the same session, so check your filter’s manual. Our filter cleaning guide compares filter manuals.

GloFish says no: “Do NOT remove the fish from the aquarium when making a partial water change.” Dr. Sanders: “Your fish can stay in the tank during cleaning, just keep an eye on them and don’t suck them up!”

Sources (23)

• Aqueon (maker), Do’s and Don’ts
• Aqueon, Cleaning Checklist
• Aqueon, Nitrogen Cycle
• Aqueon, First 60 Days
• Aqueon, Ammonia
• Aqueon, Water Quality
• API (maker), FAQs
• API, Routine Care
• API, Nitrate Test Kit
• Fluval (maker), Care Guide
• Fluval, Nitrate Kit Leaflet
• Fluval, AquaClear Manual
• Tetra (maker), Test NO3-
• Seachem (maker), Controlling Nitrate
• GloFish (maker), Water Change Sheet
• Hydor (maker), Theo Manual
• Hydor, Support Page
• UF/IFAS Extension (university), Ammonia
• Dr. Jessie Sanders (aquatic vet), Resolutions
• Dr. Jessie Sanders, Cleaning a Tank
• Practical Fishkeeping (magazine, 2009), How Often
• TFH Magazine (magazine, 2011 issue), Nitrate
• Aquarium Co-Op (retailer blog), Cleaning

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