01
Why an aquarium water change matters
A filter does not clean water in the sense most people imagine. In a cycled tank, bacteria convert toxic ammonia into nitrite and then into nitrate, but the nitrate stays in the water. Dissolved organics, hormones and leftover medication also accumulate, and the minerals that buffer pH slowly get used up. Only removing old water and adding fresh water resets all of that at once.
Skipping water changes rarely causes a visible disaster overnight. Instead, nitrate creeps up, pH drifts down, algae gets a head start and fish become more prone to disease. This slow decline is often called old tank syndrome: everything looks fine until you add new fish raised in clean water, and they die within days.
- Removes nitrate and dissolved waste that the filter cannot
- Replaces minerals that keep pH and KH stable
- Dilutes growth-inhibiting compounds and leftover medication
- Reduces the nutrients that fuel nuisance algae
- Gives you a weekly moment to spot problems early
02
How much water to change and how often
There is no single perfect number, but there is a reliable default: 20–30% once a week. That volume is large enough to keep nitrate in check in an average community tank and small enough that temperature and chemistry barely move. From there, adjust to your bioload, your plants and your test results.
The best way to decide your own schedule is to measure. Test nitrate just before a water change for a few weeks. If it stays under 20 ppm, your routine is working. If it climbs above 40 ppm, increase the volume or frequency, or look at how to lower nitrates at the source. A liquid kit from our best aquarium test kit guide is more accurate than strips for this.
| Tank type | How much | How often |
|---|---|---|
| Average community tank | 20–30% | Weekly |
| Heavily planted, lightly stocked | 25–30% | Every 1–2 weeks |
| Goldfish or large cichlids | 30–50% | Weekly, sometimes twice |
| Nano tank under 10 gallons (38 L) | 15–25% | Weekly |
| Shrimp tank | 10–15% | Weekly, very gently |
| Fry grow-out tank | 30–50% | Several times a week |
| Tank during an ammonia spike | 30–50% | Daily until ammonia is near 0 ppm |
03
What you need
A water change kit is simple and inexpensive. The one item worth choosing carefully is the siphon: a good gravel vacuum lets you drain and clean the substrate in one step, and a model with a long hose or a faucet adapter saves a lot of bucket carrying in tanks of 30 gallons and up.
- Gravel vacuum or siphon sized for your tank
- One or two buckets used only for the aquarium (never washed with soap)
- Water conditioner that neutralizes chlorine and chloramine
- Thermometer to match the new water temperature
- Algae scraper or pad for the glass
- Old towel for spills
04
How to do a water change step by step
This routine takes 15–30 minutes for most tanks. Work in the same order every time so nothing is forgotten.
- Unplug the heater and, if the water level will drop below its intake, the filter. A heater exposed to air can crack.
- Scrape algae off the glass first, so the debris gets siphoned out with the old water.
- Start the siphon and vacuum the gravel in sections, pushing the tube into the substrate and lifting it before it sucks up pebbles. Hover above sand instead of digging in.
- Stop when you have removed the planned volume. Marking the 25% level on the side of the tank helps.
- Prepare the new water. Match its temperature to the tank within about 2 °F (1 °C), using the thermometer, not your hand.
- Add water conditioner dosed for the full tank volume if you treat the tank directly, or for the bucket volume if you treat each bucket, following the label.
- Pour slowly onto a plate, a decoration or your hand to avoid blasting the substrate and uprooting plants.
- Plug the heater and filter back in, check that the filter restarts and the heater light comes on.
Treating tap water with a conditioner is not optional if your city uses chloramine. Unlike chlorine, chloramine does not evaporate if you let the water stand overnight. Our best water conditioner guide compares the main options.
05
Tap water, conditioner and temperature
Most US tap water is fine for freshwater fish once it is dechlorinated. Before relying on it, test it once for pH, KH, GH, ammonia and nitrate. Some supplies contain measurable nitrate or ammonia from chloramine, which matters if you keep sensitive species. Our water parameters cheat sheet explains which values to look for.
Temperature matching is the most common mistake. A difference of a few degrees in a 10% change is tolerated, but swapping 40% of the water with something 8 °F colder can stress fish and trigger ich. See our guide to fish tank temperature for safe ranges. If you use RO or rainwater, remineralize it before use, because pure water has no buffering capacity.
06
Common water change mistakes
Most problems linked to water changes come from doing too much at once after a long gap, not from water changes themselves.
- Changing 70–80% after months without maintenance: the sudden shift in pH and hardness can shock fish. Do several 20–25% changes over a week instead.
- Rinsing filter media in tap water: chlorine kills beneficial bacteria. Squeeze sponges in the old tank water you just removed.
- Cleaning the filter and vacuuming all the gravel on the same day: spread deep cleaning over two sessions to protect the bacterial colony.
- Forgetting the conditioner, or dosing only for the bucket when you add water straight from a hose.
- Using soap, detergent or buckets that held household chemicals.
- Topping off evaporated water without ever changing any: evaporation leaves minerals and waste behind, so they concentrate over time.
07
Special cases
New tanks. During fishless cycling, avoid big water changes unless ammonia or nitrite climbs above about 4–5 ppm, as they slow the process. With fish already in a new tank, the opposite applies: change water as often as needed to keep ammonia and nitrite low, as explained in our article on new tank syndrome.
Goldfish. Goldfish produce a lot of waste for their size. In a properly sized tank, 30–50% weekly is normal, and a strong filter does not replace it.
Shrimp. Cherry shrimp and other dwarf shrimp react badly to sudden changes in hardness and temperature. Keep changes small, match parameters closely and drip new water in slowly.
During treatment. Many medications require a water change before each new dose and a larger one at the end. Follow the product label.
08
When to change more water, and when to get help
If a test shows any ammonia or nitrite in an established tank, if fish gasp at the surface or if the water turns cloudy and smells, do an immediate 30–50% change with temperature-matched, conditioned water and repeat daily until readings return to zero. Ammonia above about 1 ppm is an emergency; our guide to ammonia poisoning in fish covers the signs and treatment.
If fish keep dying although the water tests clean, the cause may be disease, a toxin or a problem with the source water. Bring a water sample to a local fish store for a second test, and contact an aquatic veterinarian for persistent losses. For deeper maintenance tasks beyond the weekly change, see how to clean a fish tank.
Frequently asked questions
Frequently asked questions
How much water should I change in my aquarium?
For most freshwater tanks, 20–30% once a week is the right amount. Heavily stocked tanks and goldfish often need 30–50%, while lightly stocked planted tanks can manage 25–30% every two weeks. The real guide is nitrate: test before each change and adjust so it stays below 20–40 ppm.
What is the right water change frequency?
Weekly is the standard for most aquariums because it keeps nitrate and dissolved waste low without large swings in chemistry. Smaller, more frequent changes are gentler than big, rare ones. Nano tanks and shrimp tanks benefit from small weekly changes, while ammonia emergencies call for daily changes until readings are back to zero.
Can I do a 100% water change?
Avoid it with fish in the tank. Replacing all the water at once can change pH, hardness and temperature abruptly and stress or kill fish. The bacteria mostly live in the filter and substrate, so a full change does not reset the cycle, but it is still risky. In emergencies, do repeated 50% changes instead.
Do I need to remove fish during a water change?
No. Removing fish stresses them more than the water change itself. Keep the siphon away from small fish and shrimp, work calmly, and pour new water gently. Only remove fish if you are tearing down the tank or moving it.
Should I let tap water sit before adding it to the aquarium?
Letting water stand does not remove chloramine, which many US cities use, and it only slowly removes chlorine. A water conditioner works instantly on both. Letting water stand can help it reach room temperature and release excess dissolved gas, but always use a conditioner anyway.
Written and reviewed by the handsoffmyhobby editorial team according to our editorial policy.
