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Clear water and a healthy filter are the heart of a happy aquarium. If you are new to fishkeeping, it can be confusing to know how often and how much water to change, and when and how to clean your filter. This guide keeps things simple. You will learn why maintenance matters, how to set the right schedule for your tank, and step-by-step instructions that are safe for fish, plants, shrimp, and corals. Follow these tips, and your fish will be active, your water will be clear, and your tank will be stable for the long term.
Why water changes and filter cleaning matter
The nitrogen cycle in plain English
Fish and leftover food create waste. That waste becomes ammonia, which is very toxic. Good bacteria in your filter and on surfaces turn ammonia into nitrite (also toxic) and then into nitrate (less toxic). This process is called the nitrogen cycle. It keeps fish alive, but it does not remove nitrate and other dissolved waste. Over time, nitrate and other compounds build up.
Water changes remove those built-up substances. Think of them as taking out the trash. Your filter hosts the bacteria that process ammonia and nitrite, but it does not replace the need for water changes. Both jobs are necessary: bacteria make waste safer, and water changes physically remove waste.
Cleaning the filter is also important because trapped debris can rot and clog the flow. But you must clean it the right way, so you do not kill the good bacteria by accident. You will learn how below.
What dirty water does to fish
Dirty water stresses fish. High nitrate and dissolved organics can weaken immunity, fade colors, and reduce appetite. It can also cause fin rot, cloudy eyes, and sudden illnesses after small changes. Unstable pH, low oxygen, and rising TDS (total dissolved solids) also add stress.
Regular maintenance keeps your water chemistry stable. Stable water means your fish use less energy coping with stress and more energy eating, growing, and displaying natural behavior.
Water changes are not the same as topping off
When water evaporates, only pure water leaves; minerals and waste stay behind. Adding more water to bring the level up does not remove waste. Topping off is fine between water changes, but it cannot replace them. You still need to remove and replace a portion of the water to export nitrate and other pollutants.
How often should you change aquarium water?
A simple rule of thumb
Start with this easy guideline and adjust based on your tank’s test results and livestock:
– Lightly stocked tanks: 10–15% weekly or 20–30% every two weeks
– Moderately stocked tanks: 20–30% weekly
– Heavily stocked tanks or messy species: 30–50% weekly (sometimes twice weekly for very heavy loads)
“Stocking” means how many fish you have compared to the tank size and how much you feed. Messy fish (like goldfish and large cichlids) or small tanks often need more frequent changes.
Adjust by tank size and bioload
Small tanks are less stable. A 5-gallon tank can swing in temperature and waste levels quickly, so weekly or even twice-weekly changes are normal. Larger tanks hold more water and dilute waste better, so weekly changes are still best but may be less urgent if the stock is light.
Use test kits to check ammonia, nitrite, and nitrate. Ammonia and nitrite should always be 0. For most freshwater fish, aim to keep nitrate under 20–40 ppm. If nitrate is above your target before the week ends, increase the size or frequency of your water changes.
Special cases: planted tanks, shrimp, goldfish, cichlids, and bettas
– Planted freshwater tanks: Plants use nitrate. Many planted tanks do well with 20–30% weekly changes to keep nutrients balanced and avoid algae. Avoid very large changes if you use CO2, as big swings can stress plants.
– Shrimp tanks: Shrimp dislike sudden changes. Keep nitrate low (ideally under 20 ppm, many keepers prefer under 10 ppm) with smaller but frequent changes, like 10–15% once or twice weekly. Match temperature and TDS closely.
– Goldfish: They are messy and eat a lot. Plan on 30–50% weekly, and sometimes more if nitrate rises fast.
– African cichlids: Strong eaters and diggers. 30–50% weekly is common, and good filtration is essential to keep water clear and mineral levels consistent.
– Bettas: In a filtered, heated 5–10 gallon tank, 20–30% weekly is good. In very small tanks (not recommended), more frequent partial changes are necessary.
Saltwater and reef schedules
For saltwater fish-only tanks, 15–25% every 1–2 weeks works well. For reef tanks, 10–20% weekly or biweekly helps replace trace elements and keep nutrients (nitrate and phosphate) in the right range. Always mix saltwater with RO/DI water and match salinity and temperature before adding it to the tank.
How much water should you change each time?
Percentages and nitrate targets
Pick a percentage that keeps nitrate in your target range by the end of each week. Most aquariums do well with 20–30% weekly. If nitrate still climbs too high, increase the percentage or split into two changes per week.
Targets:
– Freshwater community fish: nitrate under 20–40 ppm
– Sensitive species and shrimp: under 10–20 ppm
– Reef tanks: nitrate typically 2–15 ppm, depending on coral type
Avoiding big swings
Large water changes (over 50%) can be safe if done properly, but they can also shock fish if temperature, pH, or hardness are very different. If you need to lower nitrate fast, two smaller changes in a week (for example, 30% twice) are gentler than one huge change.
Match the new water’s temperature closely. For freshwater, within 1–2°C (2–3°F) is best. For reef tanks, match temperature and salinity carefully.
Emergency water changes
If you detect ammonia or nitrite, act quickly. Do immediate 30–50% water changes daily until levels are back to zero. Reduce feeding. Make sure your filter is running well with good aeration. In emergencies, water changes save fish lives.
How to do a water change step by step
Tools you need
– Gravel vacuum or siphon hose (a “Python” style hose is very handy for larger tanks)
– Bucket used only for aquarium water (no soap residue)
– Water conditioner (dechlorinator that treats both chlorine and chloramine)
– Algae scraper or sponge
– Thermometer
– For saltwater: RO/DI water, marine salt mix, mixing container, heater, powerhead
Prepare the new water
For freshwater tanks using tap water, fill a bucket with the amount you plan to add. Add dechlorinator according to the bottle. Most conditioners can be safely overdosed a little, but follow directions. Warm the water to match the tank temperature.
If you have chloramine in your tap water (many cities do), be sure your conditioner detoxifies chloramine, not only chlorine. If you use well water, test for ammonia and metals; consider using RO water if levels are problematic.
For saltwater tanks, mix salt with RO/DI water in a separate container at least 24 hours in advance. Use a small pump and heater to mix and bring it to tank temperature. Check salinity with a refractometer and adjust before the change.
Vacuum the substrate correctly
– Gravel: Push the vacuum into the gravel and let debris rise into the tube. Lift and move section by section. Do not try to clean every inch each time. Rotate areas over multiple changes.
– Sand: Hold the siphon just above the surface. Let debris lift without digging deep. Stir the top lightly with your fingers or a chopstick to loosen waste, but do not dig into deep layers all at once.
– Planted soil or aquasoil: Do not deep vacuum. Hover above the surface to remove loose debris. Disturbing the substrate can harm roots and release nutrients into the water.
Stop siphoning when you reach your planned percentage. Keep an eye on fish to make sure none are trapped near the intake.
Refill safely
Before refilling, turn off the heater and filter to avoid running them dry. Slowly pour or pump the new water back in. For small tanks, you can place a plate or plastic bag on the substrate and pour onto it to avoid disturbing the aquascape. For large tanks, use a hose and refill gently.
Turn the filter and heater back on when the water level is back to normal. Double-check that the heater is submerged before turning it on.
Aftercare and testing
Wipe down glass and the tank rim. Check temperature. Test ammonia, nitrite, and nitrate weekly to confirm your schedule is working. If nitrate is still too high by week’s end, increase the water change size or frequency.
Filter cleaning: how often and how much
Do not kill your bacteria
The filter media holds most of your beneficial bacteria. Never scrub media in hot water or wash it under chlorinated tap water. Instead, rinse or squeeze media in a bucket of old tank water removed during the water change. This cleans out debris while keeping bacteria alive.
Avoid replacing all media at once. If you must replace something, do it in stages. Keep at least half of the old media in place so your cycle remains stable.
Cleaning schedules by filter type
– Sponge filter: Rinse and squeeze in old tank water every 1–2 weeks for heavy tanks, every 2–4 weeks for light tanks. Clean more often if flow slows.
– Hang-on-back (HOB) filter: Rinse sponges/biomedia monthly, or sooner if flow drops. Change floss/pads when clogged, but keep the sponge and biomedia. Most “cartridges” can be modified by removing carbon and keeping the fiber as reusable media.
– Canister filter: Service every 1–3 months depending on bioload. Rinse sponges and biomedia in tank water. Replace fine floss when it is clogged. Check impeller and hoses for slime that reduces flow.
– Internal power filter: Similar to HOB; clean sponges every 2–4 weeks.
– Saltwater filter socks: Change or wash 2–3 times per week to prevent nitrate spikes. Protein skimmer cups should be emptied and wiped several times per week.
How to clean each type safely
– Sponge filter: Unplug air pump. Remove sponge, squeeze it several times in a bucket of old tank water until the brown gunk is mostly out, then put it back. Keep some brown tint; it holds bacteria.
– HOB filter: Unplug. Carry the filter box to the sink with a bucket of old tank water. Rinse the sponge and biomedia in the bucket. Wipe the impeller and housing to remove slime. Do not scrub with soap. Reassemble, prime if needed, and restart.
– Canister filter: Unplug and close valves. Carry it to a sink or tub. Open and remove trays. Rinse sponges and biomedia in old tank water. Replace only clogged floss. Clean impeller and shaft with a small brush. Reassemble, fill with tank water, close securely, and prime before starting.
When to replace media
– Sponges: Replace only when they fall apart. They can last years.
– Ceramic rings/biomedia: Do not replace unless they crumble. Rinse gently when dirty.
– Filter floss/pads: Replace when clogged and cannot be rinsed clean. Many people add a thin layer of cheap floss on top as a disposable prefilter.
– Activated carbon: Only needed for removing medications, odors, or water discoloration. Replace every 3–4 weeks if used. It is optional for most freshwater tanks.
Signs your filter needs attention
If flow is slower than normal, if you hear rattling or grinding, or if you see debris blowing back into the tank, it is time to clean. A dirty filter can allow waste to decay inside and release nutrients back into the water, causing algae and cloudy water.
Putting it all together: sample maintenance plans
5-gallon betta tank (filtered, heated)
– Water change: 25–30% weekly. If nitrate climbs fast, do 15% twice weekly.
– Filter: Rinse sponge or media every 2–4 weeks in old tank water. Avoid replacing cartridges; keep the media and only replace the floss when needed.
– Extras: Wipe glass weekly. Keep feeding light to avoid waste in a small volume.
20-gallon freshwater community
– Water change: 25% weekly for a lightly stocked tank; 30% weekly for moderate stock.
– Filter: HOB or sponge prefilter. Rinse prefilter weekly and main sponge monthly. Replace floss when clogged.
– Substrate: Light vacuum each week, rotate sections so you do not uproot plants.
55-gallon African cichlid tank
– Water change: 40–50% weekly due to heavy feeding and digging.
– Filter: Canister plus prefilter on intake. Rinse prefilter weekly. Service canister monthly or every 6 weeks. Keep strong surface agitation for oxygen.
– Substrate: Hover vacuum to remove debris without collapsing rock structures.
29-gallon planted tank with CO2
– Water change: 30% weekly to reset nutrients and prevent algae.
– Filter: Rinse sponge/biomedia every 4–6 weeks or when flow drops. Do not deep vacuum aquasoil; just hover.
– Fertilizers: Dose macros and micros as per your routine after the water change. Keep CO2 steady to avoid plant melt.
10-gallon shrimp tank
– Water change: 10–15% twice weekly or 20% weekly. Match temperature and TDS closely to avoid molting issues.
– Filter: Sponge filter squeezed every 2–3 weeks. Gentle cleaning only.
– Substrate: Do not deep vacuum; let biofilm and plants handle light debris.
40-gallon reef tank
– Water change: 10–20% weekly or biweekly with well-mixed RO/DI saltwater.
– Filtration: Change filter socks 2–3 times per week. Empty and clean skimmer cup several times per week. Rinse biomedia or sponges monthly.
– Parameters: Match salinity and temperature. Keep nitrate and phosphate in target ranges for your coral types.
Common mistakes to avoid
Overcleaning the filter
Do not sterilize your filter. Hot water, tap water with chlorine, and soap will kill your bacteria. Clean gently in old tank water. If you remove too much biofilm, your tank may mini-cycle, causing ammonia or nitrite spikes.
Skipping dechlorinator
Chlorine and chloramine in tap water kill fish and beneficial bacteria. Always treat new water before it enters the tank. If your city uses chloramine, choose a conditioner that breaks chloramine and detoxifies ammonia.
Inconsistent schedules
Skipping water changes for weeks and then doing a huge one can stress fish more than small, regular changes. Consistency keeps chemistry stable. Set reminders on your calendar so maintenance becomes a habit.
Cleaning everything the same day after a large change
You can clean the filter and do a water change on the same day if you are gentle. But avoid deep-cleaning all media and doing a very large water change at once, especially on a young tank. If you plan a big water change (over 40%) and a heavy filter clean, consider staggering them by a few days to reduce risk.
Neglecting prefilters, intakes, and hoses
Gunk collects on intake sponges and inside hoses, reducing flow and oxygen. Rinse prefilters weekly and brush hoses every 1–2 months. Good flow keeps your whole system healthier.
Testing and tracking: make decisions with data
What to test and why
Test ammonia, nitrite, and nitrate weekly, especially in new tanks. You want ammonia and nitrite at zero and nitrate in your chosen target range. In planted tanks, you may also test phosphate and potassium for nutrient balance. For shrimp, track TDS, GH, and KH for stable molting conditions. Reef tanks should also test salinity, alkalinity, calcium, and magnesium.
Choosing test kits
Liquid test kits are more accurate than most strips, but strips are fast and can be useful for a quick check. Whichever you choose, test consistently and write down results. If nitrate creeps up week after week, increase your water change size or reduce feeding.
Interpreting results
– Ammonia or nitrite above 0: increase water changes, reduce feeding, check filter flow, and confirm that your media is not over-cleaned.
– Nitrate above target: increase your weekly percentage, do an extra midweek change, or improve mechanical filtration to catch debris before it breaks down.
– pH swings: check KH; low KH can cause unstable pH. Use buffers or choose water sources and substrates that support stable KH.
Extra tips for different setups
Heavily planted or aquascaped tanks
Trim plants before water changes so you can remove floating leaves and detritus easily. If you dose fertilizers, many aquarists dose after the water change to reset levels. Keep the filter intake covered with a sponge to protect shrimp and catch debris.
Sand beds and anaerobic spots
Very deep, compacted sand can develop low-oxygen pockets. Avoid deep stirring of old beds all at once. Instead, gently rake small sections at each water change. Malaysian trumpet snails can help keep sand aerated in freshwater tanks.
Quarantine and hospital tanks
These often need frequent water changes because biofiltration may be minimal and medicines can affect bacteria. Daily small changes are common during treatments. Always match temperature and redose medications as needed after changes.
FAQ: quick answers
Can I change 100% of the water?
In most cases, no. A 100% change can shock fish and remove too much beneficial bacteria from the water column and surfaces. Only do very large changes in emergencies and even then, match temperature and parameters carefully. Multiple moderate changes are safer than one huge change.
My water is cloudy after a change. What happened?
Cloudiness is often a bacterial bloom or disturbed debris. Make sure the filter is running well and avoid overfeeding. It usually clears in a day or two. If the cloudiness is white and persistent, test ammonia and nitrite. If it is green, you may have an algae bloom; reduce light and nutrients.
Do I need to vacuum a planted substrate?
Only lightly. Hover to pick up loose debris without digging deep. Deep vacuuming can harm roots and release nutrients that fuel algae.
Is activated carbon required?
No. Carbon is useful for removing medication, odors, or tannins. It is optional for most freshwater aquariums. If you use it, replace it every 3–4 weeks.
What if my tap water is very different from my tank water?
Large changes might cause stress. You can do smaller, more frequent changes to reduce swings, blend RO water with tap to adjust hardness, or condition water to stabilize KH and pH. Always match temperature.
Can I do maintenance with fish in the tank?
Yes. Most water changes and filter cleanings are done with fish in the tank. Move slowly, keep hands clean, and avoid sudden movements. Always unplug the heater before lowering the water level and plug it back in only when it is fully submerged again.
A simple maintenance checklist
Weekly
– Test ammonia, nitrite, nitrate
– Change 20–30% of the water (adjust for your tank’s needs)
– Lightly vacuum substrate or hover vacuum for sand/planted tanks
– Wipe glass and check equipment
Biweekly to monthly
– Rinse filter sponges/biomedia in old tank water
– Replace clogged floss
– Check impeller and hoses for slime and debris
As needed
– Increase water changes during heavy feeding, after adding fish, or if tests show rising nitrate
– Replace media only when worn out, not on a schedule
Conclusion
Water changes and filter cleaning work together to keep your aquarium safe and stable. Your filter’s bacteria convert toxic ammonia and nitrite into less harmful nitrate. Regular water changes remove nitrate and other dissolved wastes that filters cannot remove on their own. Choose a schedule that fits your tank’s size and bioload: most tanks thrive with 20–30% weekly changes, plus gentle filter cleaning every few weeks. Rinse media in old tank water, avoid replacing all media at once, and test weekly to confirm that your plan is working. With steady, simple maintenance, your water stays clear, your fish stay healthy, and your aquarium becomes a calm, reliable piece of living art.