We are reader supported. When you purchase through links on our site, we may earn an affiliate commission. Also, as an Amazon affiliate, we earn from qualifying purchases.
Clean, stable water is the heart of a healthy aquarium. New fish keepers often ask the same question: how often should I change the water? The truth is, there is no single schedule that fits every tank. Your tank’s size, fish species, plants, filter, and even how much you feed all change the answer. This guide explains how to set the right water change routine for your aquarium, why it matters, and how to do it safely and easily. By the end, you will know when, how much, and how often to change water—without stressing your fish or yourself.
Why Water Changes Matter
The Nitrogen Cycle in Simple Terms
Fish produce waste. Leftover food also breaks down into waste. Helpful bacteria in your filter convert toxic ammonia (from waste) into nitrite (also toxic), and then into nitrate (less toxic). This process is called the nitrogen cycle. Ammonia and nitrite should always be at 0 ppm in a mature, cycled tank. Nitrate builds up slowly and should be kept low. Water changes remove nitrate and other waste that filters cannot remove.
What Water Changes Remove
Water changes lower nitrate, dissolved organic compounds (DOC), and other pollutants that cause dull water, algae, and stress. They remove excess hormones and minerals that can build up over time. Regular changes keep the water clear, reduce odors, and improve fish color and behavior.
What Water Changes Add
Fresh tap or remineralized water restores essential minerals like calcium and magnesium. It also refreshes carbonate hardness (KH), which helps keep pH stable. Many fish do better with steady mineral levels. Without water changes, pH can drop and mineral levels can become too low or too high, causing stress.
How Often Should You Change Aquarium Water?
The Short Answer
For most freshwater community tanks: change 20–30% of the water every week. This keeps nitrate below 40 ppm, stabilizes pH and hardness, and maintains water clarity. However, adjust this schedule based on your stocking level, plant growth, feeding habits, and test results.
By Tank Type: Simple Guidelines
Freshwater community (tetras, guppies, rasboras, livebearers, peaceful barbs): 20–30% weekly. Heavier stocking or overfeeding may need 30–50% weekly.
Low-tech planted tanks (no CO2, moderate light): 20–30% weekly. Plants use nitrate, so you may get away with 15–20% weekly if nitrate stays under 20–30 ppm.
High-tech planted tanks (CO2, high light, fertilizers): 30–50% weekly. Large changes reset nutrient levels and reduce algae risk.
Nano tanks (under 10 gallons): 20–40% twice per week, or 30–50% weekly. Small volumes swing faster; frequent small changes are safer.
Betta in a heated, filtered 5–10 gallon tank: 25–30% weekly. In unfiltered bowls (not recommended), 50% every 2–3 days, plus full clean weekly.
Goldfish tanks: 30–50% weekly. Goldfish produce heavy waste. More frequent changes for small tanks or young fish that eat often.
African rift lake cichlids (Malawi, Tanganyika): 25–40% weekly. Keep nitrate under 20–30 ppm to encourage color and breeding behavior.
Shrimp tanks (neocaridina, caridina): 10–20% weekly, carefully matched temperature and TDS. Many keepers prefer small, frequent changes to avoid shock.
Brackish tanks (puffers, mollies in brackish): 20–30% weekly. Mix brackish water to the same salinity before adding.
Saltwater fish-only (FOWLR): 15–25% weekly or 25–30% every two weeks, depending on nitrate and phosphate buildup.
Reef tanks: 10–20% weekly. Consistency is key. Water changes help replenish trace elements and balance nutrients.
Quarantine or hospital tanks: 25–50% as needed daily or every few days, based on test results and medication directions.
How Much Water to Change Each Time
Choosing a Safe Percentage
10–15%: Good for very stable tanks or delicate species that dislike sudden changes. Works well between larger changes.
20–30%: The standard for most tanks. Safe, effective, and easy to manage temperature and parameter matching.
40–50%: Useful for overstocked tanks, heavy feeding, or when nitrate is high. Match temperature and dechlorinate carefully to avoid stress.
Over 50%: Usually not needed for normal maintenance. Large changes can be risky if parameters (temperature, pH, hardness, salinity) are not matched closely. For emergencies only.
What Affects Your Water Change Schedule
Stocking Level and Fish Size
More fish means more waste. Big, messy eaters like goldfish, plecos, cichlids, and loaches often require larger or more frequent changes. Lightly stocked tanks can go longer between changes, as long as nitrate stays low.
Feeding and Food Type
Overfeeding is the fastest way to foul water. Feed small amounts your fish finish within a few minutes. Frozen and live foods can cause more waste than pellets, so tune your schedule if you feed rich foods often.
Filter Size and Maintenance
A filter with strong biological media helps convert ammonia and nitrite to nitrate, but it does not remove nitrate. Rinse filter media in tank water (never tap water) every 2–4 weeks to remove sludge without killing beneficial bacteria.
Plants and Algae Balance
Healthy plants absorb ammonia and nitrate. The more plants you have, the slower nitrate rises. Fast growers like hornwort and water sprite help a lot. If algae is growing, consider larger or more frequent changes while you correct light and nutrients.
Substrate and Cleaning
Gravel and sand trap debris. Regular vacuuming during water changes prevents pockets of rotting waste. In planted tanks, lightly hover the vacuum to avoid damaging roots.
Tap Water Chemistry
Hardness (GH), carbonate hardness (KH), and pH vary by city. Low KH water is prone to pH swings and benefits from steady changes and possible buffering. High nitrate in tap water may require a different approach, like using RO/DI water and remineralizing.
Temperature and Season
Fish are sensitive to temperature drops. In cold seasons, match the replacement water temperature within 1–2°C (2–3°F). In hotter months, avoid sudden swings. Stable temperature reduces stress during changes.
Testing: The Key to a Personalized Schedule
What to Test
Ammonia (NH3/NH4): should always be 0 ppm in a cycled tank.
Nitrite (NO2): should always be 0 ppm.
Nitrate (NO3): aim for under 40 ppm for most fish, under 20 ppm for sensitive fish, and under 10–20 ppm for reefs and shrimp.
pH and KH: check monthly or when fish act stressed.
Salinity (for brackish or marine): use a hydrometer or refractometer and keep it stable.
Target Ranges for Most Freshwater Fish
Ammonia: 0 ppm. Nitrite: 0 ppm. Nitrate: 5–30 ppm (lower is better, but some nitrate is fine). pH: stable within your fish’s preferred range. KH: enough to maintain stable pH (often 3–8 dKH).
How to Use Test Results
Test your water before and 24 hours after a water change. If nitrate creeps above 40 ppm before your next scheduled change, increase the frequency or volume. If nitrate stays under 20 ppm consistently, you may reduce the volume slightly, but keep a weekly schedule to maintain stability.
Step-by-Step Water Change Guide
Tools You Will Need
Siphon or gravel vacuum. Bucket reserved for aquarium use only. Dechlorinator (that treats chlorine and chloramine). Thermometer. Algae scraper or sponge. For saltwater or brackish: salt mix, refractometer or hydrometer, powerhead, and heater for mixing water.
Freshwater Water Change Steps
1) Unplug heaters and equipment as needed for safety. Keep filters running if their intake stays underwater; otherwise turn them off temporarily.
2) Clean the glass with an algae scraper. This prevents scraped algae from staying after the change.
3) Siphon 20–30% of the water into a bucket. Vacuum the substrate lightly to remove debris. In planted tanks, hover above the substrate to avoid root damage.
4) Treat tap water with dechlorinator according to the bottle. If you have chloramine in your water, use a conditioner that handles chloramine and binds ammonia.
5) Match temperature. Use a thermometer to keep replacement water within 1–2°C (2–3°F) of the tank.
6) Slowly pour the new water back into the tank, aiming at a plate or decor to avoid disturbing the substrate.
7) Restart equipment, confirm heater and filter are working, and wipe up spills. Test ammonia, nitrite, and nitrate weekly to track progress.
Saltwater or Brackish Water Change Steps
1) Use RO/DI water if possible. Add marine or brackish salt mix to a separate container. Never add dry salt directly to the tank.
2) Mix with a powerhead and heat to match tank temperature. Aerate and mix for several hours (ideally 12–24 hours) to stabilize pH and oxygen.
3) Check salinity with a refractometer or hydrometer. Adjust until it matches the display tank.
4) Siphon out 10–20% (reef) or 15–25% (FOWLR). Clean the glass and gently turkey baste detritus off rocks before siphoning.
5) Add the prepared saltwater slowly. Restart equipment and verify skimmer function, as it may behave differently right after a change.
Fertilizers and Conditioners
If you dose fertilizers in a planted tank, add them after the water change to hit the correct weekly target. For blackwater tanks, add tannins (like botanicals or liquid extracts) after the change to maintain your desired tint. If your tap is very soft, consider remineralization salts to keep GH and KH in a safe range.
Special Cases and Tips
New Tanks and Cycling
During a fish-in cycle, change water as often as needed to keep ammonia and nitrite under 0.25–0.5 ppm. This may mean daily or every other day changes at first. Water changes do not stop the cycle; they keep fish safe while bacteria grow.
Fry and Shrimp
Fry and shrimp are sensitive to sudden changes. Do smaller, more frequent water changes (10–15% several times a week), and match temperature and TDS closely. A drip method for refilling can reduce shock.
Soft Water, Blackwater, and RO Users
If your water is very soft or you use RO/DI, you may need to remineralize to maintain stable GH and KH. Use a reliable remineralizer and test KH to prevent pH crashes. For blackwater setups, frequent small changes with re-added botanicals keep the chemistry steady.
Well Water and Chloramine
Well water may contain metals or gases. Test regularly and aerate before use. City water often has chloramine, which does not evaporate. Always use a conditioner that neutralizes both chlorine and chloramine.
Vacations and Busy Weeks
Before you leave, do a larger change (30–40%), clean the filter intake and glass, and reduce feeding. Consider an automatic feeder set to a small amount. Ask a trusted friend to top off evaporated water and feed sparingly. After you return, test and adjust with a normal change.
Common Mistakes to Avoid
Changing Too Little or Too Rarely
Only topping off water is not a water change. Topping off replaces evaporated water, but it does not remove waste. Make sure to remove and replace water regularly to keep nitrate and DOC low.
Changing Too Much at Once Without Matching Parameters
Huge water changes can shock fish if temperature, pH, GH, KH, or salinity are off. Avoid over 50% unless it is an emergency and you can match parameters closely.
Not Using Dechlorinator
Chlorine and chloramine can kill beneficial bacteria and harm fish and invertebrates. Always treat replacement water properly.
Cleaning Filter Media with Tap Water
Rinse sponges, pads, and bio-media in removed tank water, not under the tap. Tap water can contain chlorine that kills your beneficial bacteria.
Overfeeding and Skipping Gravel Vacuuming
Extra food becomes waste. Feed less and vacuum the substrate during each change to keep the bottom clean and oxygen flowing.
Ignoring Signs from Fish and Plants
Gasping at the surface, clamped fins, pale colors, or algae blooms are signals to test and perform a water change. Watch your tank daily to catch problems early.
Signs You Need a Water Change Now
Water Clarity and Odor
Cloudy water, a musty smell, or a greasy film on the surface often means organics are high. A 30–40% change plus filter maintenance usually helps.
Test Results Out of Range
Ammonia or nitrite above 0 ppm requires immediate action. For nitrate above 40–50 ppm, plan larger or more frequent changes until levels come down.
Fish Behavior and Appearance
Fast breathing, hiding, loss of appetite, or unusual flicking/scratching can signal poor water quality. Do a change and test to confirm the issue.
Troubleshooting After a Water Change
Fish Gasping at the Surface
Possible causes include temperature shock, chlorine/chloramine, or a swing in pH. Check temperature, add more dechlorinator if needed, increase aeration, and verify KH stability.
Cloudy Water
A bacterial bloom can follow large changes or substrate cleaning. Keep filters running, avoid overfeeding, and the cloudiness should clear in a few days. Regular, moderate changes help prevent this.
pH Swings
Low KH allows pH to swing after changes. Use a buffer or choose smaller, more frequent changes. Test KH and aim for at least 3–4 dKH in most community tanks for stability.
Maintenance Calendar Example
Weekly Tasks
Change 20–30% of the water. Vacuum part of the substrate. Clean glass. Trim dead plant leaves. Check temperature and equipment. Test nitrate and record results to watch trends.
Every 2–4 Weeks
Rinse filter sponges and mechanical media in tank water. Do not replace all media at once. Rotate cleanings to protect your bacterial colony.
Monthly
Test pH, KH, and GH. Deep vacuum a different section of the substrate if the tank is not heavily planted. Check hoses, impellers, and air stones for clogs.
Seasonal
Review your schedule. If nitrate stays low, you are on track. If it climbs between changes, increase the frequency or volume. Adjust heater settings as room temperatures change.
Frequently Asked Questions
Does topping off count as a water change?
No. Topping off only replaces evaporated water. It does not remove waste. You must remove and replace a portion of the tank water to lower nitrate and DOC.
Can I change 100% of the water?
Not for routine maintenance. Full changes can crash parameters and stress fish. Use 20–30% weekly for normal care. Reserve large changes for emergencies, matching parameters very carefully.
Do plants mean I never need water changes?
No. Plants help, but they do not remove all waste or restore minerals. Even well-planted tanks benefit from regular changes to keep nutrients balanced and replenish minerals.
Is distilled or RO/DI water safe to add directly?
Not by itself for freshwater fish. Pure water lacks minerals and can crash KH and pH. Remineralize RO/DI water before use, or mix it with tap water to reach stable GH and KH.
How do I handle chloramine?
Use a conditioner that neutralizes chloramine and binds the resulting ammonia. Follow the dosage on the label. Many modern conditioners handle both chlorine and chloramine.
Will water changes remove my beneficial bacteria?
No. Most beneficial bacteria live on surfaces: filter media, substrate, decor. As long as you do not sterilize these and you use dechlorinator, normal changes are safe.
Why does nitrate stay high even with changes?
Check feeding, stocking levels, and filter cleanliness. Increase change volume or frequency. Consider adding more plants or upgrading filtration. Test your tap water; if nitrate is high out of the tap, you may need RO/DI water mixed with remineralizer.
Putting It All Together: A Simple Plan
Start with a Baseline
Begin with 20–30% weekly changes for most freshwater tanks. For nano tanks, consider two smaller changes per week. For reefs, 10–20% weekly is solid. Keep a log of dates, volumes, and test results.
Tune by Testing
Test nitrate each week before your scheduled change. If nitrate is above 40 ppm, increase the frequency or the percentage. If nitrate stays under 20 ppm, you are on track. Always keep ammonia and nitrite at 0 ppm.
Keep It Consistent
Fish thrive on routine. A consistent schedule beats a perfect but irregular one. Set reminders, keep tools ready, and make it part of your weekly routine. Small, steady steps prevent big problems.
Conclusion
Clean Water, Healthy Fish, Happy You
Water changes are the simplest and most powerful tool you have to keep your aquarium healthy. They remove waste that filters cannot, refresh minerals, stabilize pH, and keep your fish vibrant. Start with 20–30% weekly for most freshwater tanks, then fine-tune with testing and observation. Match temperature, dechlorinate, and clean gently. For special setups—planted, shrimp, brackish, or marine—adjust the plan to suit your tank’s needs, but keep your routine steady. With a clear schedule and a few good habits, you will enjoy clear water, strong plant growth, and active, colorful fish for years to come.