How much water to change in a fish tank (and how often)

Change about 25–30% of the water once a week in most freshwater aquariums, then adjust by testing nitrate: keep it under 40 ppm, and ideally at 20 ppm or less. Lightly stocked planted tanks can often go 1–2 weeks between changes, while heavy waste producers such as goldfish and oscars often need 30–50% weekly. Treat every batch of new water with a dechlorinator that also handles chloramine, following the label, and match the tank's temperature.

Most freshwater tanks do well with a 25–30% water change once a week. Treat that as a starting point, not a rule. Published advice runs from about 10% a week for lightly stocked tanks to 30% or more for heavy stocking, so the right amount for your tank is whatever keeps nitrate low and the water chemistry steady. Two weeks of nitrate tests will tell you what that is.

Typical water change schedules

Tank	How much	How often
Lightly stocked, well planted	20–25%	Every 1–2 weeks
Typical community tank	25–30%	Weekly
Heavily stocked, or big waste producers (goldfish, common plecos, large cichlids)	30–50%	Weekly, or split into two changes
Nano tank, 5 gallons (19 L) or less	25–30%	At least weekly; small volumes foul fast
Cycling with fish in the tank	25–50%, more if readings are high (if your tap water reads 0, a 50% change halves them)	Test daily and change water the same day ammonia or nitrite reads 0.25 ppm or higher (see how to cycle a fish tank)

Aim for nitrate under 40 ppm in most tanks and 20 ppm or less if you can. Sensitive fish such as otocinclus do best with nitrate kept low. Invertebrates such as dwarf shrimp are more sensitive still, and under 15 ppm is a common target for them. If nitrate keeps climbing even with weekly changes, the tank may hold more fish than it can handle. Check your stock with the stocking calculator.

Why fish tanks need water changes

A cycled filter turns toxic ammonia and nitrite into nitrate, but it does not make the water clean. Three things build up that only fresh water fixes.

Nitrate. Filter bacteria turn ammonia from fish waste into nitrite and then nitrate (the nitrogen cycle). Nitrate is much less toxic, but it keeps rising. In closed systems with little or no water exchange it can pass 250 ppm. Long exposure to high levels is linked to stress, weaker immunity and poor breeding, and UF/IFAS notes that newer research suggests nitrate may be more harmful than once believed. Plants absorb some of it. Water changes remove the rest.
Lost minerals and buffering. Nitrifying bacteria produce acid, which slowly uses up carbonate hardness (KH). As KH falls, pH drifts down and can crash. The bacteria that process ammonia slow down as the water turns acidic, and below about pH 6.0 they may stop. Fresh tap water puts back the KH, GH and minerals it contains.
Dissolved waste and phosphate. Leftover food, fish waste and decaying plant matter release dissolved organic compounds, which can yellow the water, and phosphate, which feeds algae. Hobby kits can measure phosphate but not most of the other compounds, and a water change dilutes them all at once.

Topping off evaporation is not a water change. Only pure water evaporates. The minerals and waste stay behind, so a tank that only gets topped off becomes more concentrated every week.

How many gallons to change by tank size

Label sizes are nominal and roughly match the volume of the glass box. Substrate, rock and the gap below the rim mean the tank holds less water than that, so these figures slightly overshoot. The aquarium volume calculator gives your real volume.

Tank size	25% change	30% change
5 gal (19 L)	1.25 gal (4.7 L)	1.5 gal (5.7 L)
10 gal (38 L)	2.5 gal (9.5 L)	3 gal (11.4 L)
20 gal (76 L)	5 gal (19 L)	6 gal (23 L)
29 gal (110 L)	7.25 gal (27 L)	8.7 gal (33 L)
40 gal (151 L)	10 gal (38 L)	12 gal (45 L)
55 gal (208 L)	13.75 gal (52 L)	16.5 gal (62 L)
75 gal (284 L)	18.75 gal (71 L)	22.5 gal (85 L)
125 gal (473 L)	31.25 gal (118 L)	37.5 gal (142 L)

Water weighs about 8.34 lb per gallon, so a full 5-gallon (19 L) bucket is around 42 lb (19 kg). For tanks over about 40 gallons, a long hose that drains to a sink or outside and refills from the tap saves a lot of lifting. If you refill straight into the tank, add the dechlorinator first and dose it the way the label says for direct use, since some products ask you to dose for the whole tank volume.

Measure once so you hit the same amount every time: put tape on the glass at the 25% line, or count buckets.

The nitrate math behind water changes

One change

If your tap water has no nitrate, a water change removes the same share of nitrate as the share of water you replace. Change 25% and nitrate drops by a quarter: 40 ppm becomes 30 ppm. Change 50% and it halves.

Several changes in a row

Each change removes a share of what is left, not of the starting amount. With nitrate-free tap water, the share of nitrate left after n changes of fraction p is (1 − p)^n. That is why two 50% changes remove 75% of the nitrate, not 100%.

Change size	After 1 change	After 2	After 3	After 4
10%	90% left	81%	73%	66%
20%	80% left	64%	51%	41%
25%	75% left	56%	42%	32%
30%	70% left	49%	34%	24%
50%	50% left	25%	13%	6%

Example: nitrate is 80 ppm and you want it at 20 ppm or less. With 25% changes it takes five (80 → 60 → 45 → 34 → 25 → 19 ppm). With 50% changes it takes two (80 → 40 → 20 ppm). These figures ignore the nitrate your fish add between changes, so retest afterwards.

Where nitrate settles on a weekly routine

Nitrate rises by roughly the same amount each week (call it R). If you change the same share p every week, nitrate settles into a steady pattern. Right before each change it peaks at about:

peak nitrate = tap nitrate + R ÷ p

Right after each change it sits R lower. For a tank that gains 10 ppm a week, with nitrate-free tap water:

Weekly change	Nitrate before the change	Nitrate after the change
10%	100 ppm	90 ppm
20%	50 ppm	40 ppm
25%	40 ppm	30 ppm
30%	33 ppm	23 ppm
50%	20 ppm	10 ppm

Two things follow:

Small changes can fall behind. A 10% weekly change lets this tank sit at 90–100 ppm indefinitely, even though you never miss a week. In a lightly stocked tank where nitrate rises only 1–2 ppm a week, the same 10% keeps it at 10–20 ppm. Your weekly rise decides which case you are in.
Splitting the change doesn’t change the peak. Two 15% changes a week give the same 33 ppm peak as one 30% change, with smaller swings in between. Pick whichever suits your week.

Your tap water sets the floor

You can’t dilute nitrate below the level in your tap water. In the US, public water systems may supply up to 10 mg/L of nitrate measured as nitrogen. That is about 44 ppm on the nitrate (NO3) scale that common liquid hobby kits report. A kit that reads nitrate-nitrogen (NO3-N) shows a number 4.4 times lower, so check which one yours uses. Private wells aren’t covered by that limit and can be higher. If your tap reads 10 ppm, the same 25% weekly routine settles at 50 ppm instead of 40 ppm.

Test your tap water once so you know your floor. If it is high, more plants, lighter stocking and less food all help, and a nitrate-removing resin or RO filter can treat the new water. Pure RO water has almost no minerals, so mix it with tap water or add a remineralizing product, following its label, before it goes in the tank.

Find your tank’s weekly nitrate rise

Test nitrate right after a water change.
Test again a week later, just before the next change.
The difference is your weekly rise, R. Repeat for a second week to confirm it.

Put R, your tap nitrate and your change size into the water change calculator. It shows where your tank will settle and how many changes it takes to reach a target.

How to do a water change, step by step

You need a gravel vacuum (siphon), a bucket that is only ever used for the aquarium, a thermometer and a dechlorinator.

Test first. Write down nitrate, and pH if you track it. Weekly tests for the first month show you how your tank behaves.
Unplug the heater. Do this before the level drops, so the heater never runs while exposed to air. Switch off a hang-on-back filter too if the water will fall below its intake.
Wipe the glass. Clean algae off the glass before you siphon, so the loose bits leave with the old water.
Siphon and vacuum. Push the vacuum into the gravel, let the dirt lift, then move on before the gravel follows it out. Clean about a third of the substrate each time and rotate sections. Over sand and rooted plants, hover just above the surface.
Stop at your target. Use your tape line or bucket count.
Rinse filter media only if it’s due. Swish sponges in the bucket of old tank water, never under the tap, when flow has slowed or at your regular monthly clean. How to clean a fish tank covers what to rinse and what to leave alone.
Prepare the new water. Match the tank temperature using a thermometer, then add dechlorinator as the label directs.
Refill gently. Pour onto a rock, a plate or your hand so the stream doesn’t dig up the substrate or uproot plants.
Restart and check. Plug the heater and filter back in, top up to the normal level, and check the temperature again after 30 minutes.

Getting the new water right

Temperature

Match the new water to the tank as closely as you can with a thermometer. Within about 2 °F (1 °C) is a sensible target. Sudden temperature swings stress fish, and University of Florida fish health specialists note that ich (white spot) outbreaks often appear when water temperature is changing quickly.

There is one deliberate exception. Corydoras, otocinclus and some other catfish often spawn after a large, slightly cooler water change. Breeders use this on purpose, but only with healthy, well-fed fish, and it is not something to try on stressed or sick ones.

Dechlorinator

Most city tap water contains chlorine or chloramine to make it safe to drink. Even low levels can make fish sick, and they can damage the bacteria in your filter. Chlorine slowly gasses off from standing water, but chloramine does not, and suppliers can switch between the two, so the safe habit is to treat every water change, including small top-offs.

It works fast. A dechlorinator neutralizes chlorine and chloramine within a few minutes.
Dose by the label. Some products are dosed for the volume of new water. Others say to dose for the whole tank if you add them straight to the aquarium. Read yours, and don’t overdose beyond what the label allows.
Chloramine leaves ammonia behind. Breaking chloramine releases a small amount of ammonia. A cycled filter handles it, and some conditioners also bind ammonia temporarily, for up to a day or two depending on the product, while the bacteria catch up.
Well water usually has no chlorine, but it can carry nitrate or metals. Test it before you rely on it.

pH and hardness

Test your tap water and tank water side by side. Let a glass of tap water stand for about 24 hours before testing its pH, because fresh tap water can carry dissolved gases, mainly CO2, that make it read differently from how it settles in the tank. A gap of 0.5 pH or more between tank and tap is a warning sign in an older tank, because its buffering may have run out. Some tanks differ on purpose, for example with driftwood or CO2 injection. If yours has drifted, close the gap slowly with smaller, more frequent changes rather than one big one. Aquarium water parameters explains safe ranges for pH, KH and GH.

When to change more water (or less)

Situation	What to do
Nitrate above 40 ppm	Change 25–30% every 2–3 days until it is back under 40 ppm. Then find the cause: overfeeding, overstocking or a dirty filter.
Ammonia or nitrite above zero in an established tank	Change 25–50% straight away and retest the next day. Repeat until both read zero, and look for the cause, such as a dead fish, overfeeding or an overcleaned filter.
Tank neglected for months	Change 10–25% every day or two until nitrate and pH are back near normal. Don’t do one huge change.
Lightly stocked planted tank with nitrate under 10 ppm	Keep doing about 25% every 1–2 weeks to replace minerals and remove dissolved waste.
Fish being treated with medication	Follow the product label, or your aquatic vet’s instructions for prescribed treatments. Many say when to change water during or after treatment, and some medicines harm filter bacteria, so test ammonia while you treat.
Fish gasp, hide or die after a water change	Check that the new water was dechlorinated and temperature-matched, then test ammonia, nitrite and pH. If fish keep getting worse, contact an aquatic vet.

Neglected tanks need extra care because of old tank syndrome: as KH runs out, pH sinks and nitrate builds, and the fish adapt to water that has drifted a long way from your tap water. Low pH also keeps most ammonia in its far less toxic form (ammonium). A sudden big change raises the pH, which turns more of that ammonium into toxic ammonia just as the fish face a jump in chemistry. Small daily changes let everything adjust together. If fish are already gasping or sick, test ammonia first and contact an aquatic vet.

Common water change mistakes

Mistake	Why it hurts	Do this instead
Changing 100% of the water, or stripping the tank down	Temperature, pH and hardness all swing at once. Scrubbing gravel and decor under the tap also removes the bacteria living on them.	Do partial changes. Keep routine changes to 50% or less.
Using untreated tap water	Chlorine and chloramine can harm fish and damage filter bacteria, which can bring an ammonia spike a few days later.	Dechlorinate every time, including top-offs.
Washing buckets with soap or household cleaners	Residue is toxic to fish.	Keep a bucket only for the aquarium and rinse it with water only.
Judging by how clear the water looks	Nitrate and dissolved waste are invisible. Crystal-clear water can test at 80 ppm.	Test nitrate before the change.

Gear mentioned in this guide

Gravel vacuum (siphon) (opens Amazon in a new tab)Cleans the gravel and does the water change in one go. How to choose
Water conditioner (dechlorinator) (opens Amazon in a new tab)Removes chlorine and chloramine from tap water. Seachem Prime is a popular, concentrated choice. How to choose
A bucket used only for the aquarium (opens Amazon in a new tab)Never use one that has held soap or cleaning products. How to choose
Aquarium thermometer (opens Amazon in a new tab)Check it daily: heaters can fail stuck on or off. How to choose

Product links go to Amazon search results. As an Amazon Associate we earn from qualifying purchases, at no extra cost to you. Disclosure.

Frequently asked questions

How much water should I change in my fish tank each week?

For most community tanks, change 25–30% once a week. Test nitrate just before the change. If it is over 40 ppm, change more or more often; if it stays well under 20 ppm in a lightly stocked tank, every 10–14 days can be enough.

Is a 50% water change safe?

Yes, in a tank you look after regularly, as long as the new water is dechlorinated and close to the tank's temperature. Keep routine changes at 50% or less. In a tank that has gone months without changes, do small changes of 10–25% every day or two instead, because one big change can shock fish that have adapted to the drifted water.

Do I need to take the fish out during a water change?

No, leave them in. Catching and netting fish is more stressful than a partial water change, and a gravel vacuum works fine around them. Only move them to a filtered holding tank for a full teardown, for example when the substrate smells of rotten eggs.

Can I let tap water sit out instead of using a dechlorinator?

Only if your supply uses chlorine, which leaves standing water within a few days (1–5 days is the usual range). Many water suppliers use chloramine instead, which does not leave by standing, and suppliers can switch between the two, so a dechlorinator that treats chloramine is the safe default.

Do water changes remove the beneficial bacteria?

Hardly any. Nitrifying bacteria live on the filter media, substrate and decor, not floating in the water, so a partial change leaves the cycle intact. Untreated tap water and replacing all the filter media at once are what harm them.