Aquarium water changes
The single most important ongoing task in fishkeeping: how often, how much, and how to actually do it without harming your fish.
Published 3 June 2026 · Last updated 21 June 2026 · 11 min read
Why water changes matter
A cycled aquarium handles ammonia and nitrite for you: beneficial bacteria convert both into nitrate, which is much less toxic. But nitrate does not just disappear. Without intervention it builds up indefinitely. Over weeks and months, three things accumulate alongside it: dissolved organic compounds from waste and uneaten food, depleted trace minerals (calcium, magnesium, carbonate hardness), and a slow drift in pH as the water's buffering erodes.
The solution is not a filter, a chemical, or a plant. It is replacing some of the old water with new water. A water change removes built-up nitrate and waste, replaces the minerals your fish and plants need, and resets your tank's chemistry toward what comes out of the tap. No equipment substitutes for this. Skipping water changes is the most common reason long-running tanks decline.
How often you should change water
The honest answer is that it depends on your stocking, plants, filtration, and source water. But for the overwhelming majority of community freshwater tanks, the right answer is:
Once a week.
This is the default that works for nearly every standard setup. Weekly is frequent enough to keep nitrate well below harmful levels (usually under 20 ppm), to stop dissolved organics building up, and to keep your parameters stable. It is also infrequent enough not to stress you or your fish.
Some situations call for adjustment:
- Heavily stocked tanks (check your level with the stocking calculator) may need changes twice a week, or larger weekly changes (40 to 50% rather than 25%).
- Heavily planted tanks with light stocking often need less: every 10 to 14 days can be enough, because plants use nitrate directly.
- Shrimp-only and breeding tanks usually do better with smaller, more frequent changes (10 to 15% twice a week) to keep parameters very stable.
- New tanks in their first few months are more fragile and benefit from a rigid weekly schedule even if testing suggests less is needed.
The signal that you are doing too few water changes is rising nitrate. If your nitrate stays under 20 ppm with weekly 25% changes, your schedule is fine. If it creeps above 40 ppm between changes, increase the frequency or the amount.
How much to change at once
For routine maintenance, the answer is 25% of your tank volume. This is the standard recommendation for a reason: it removes a meaningful amount of nitrate (a 25% change removes roughly 25% of dissolved waste) while being small enough not to shock your fish with sudden changes.
To work out 25%, you need your tank's actual water volume, which is not the same as its stated capacity. A "100-litre" aquarium typically holds around 85 to 90 litres once you account for substrate, decor, and the gap below the rim; our tank volume calculator estimates it from your tank's dimensions. Most keepers learn their real volume by measuring how much water they remove during a change.
Larger changes are sometimes useful. 50% changes are common after diagnosing a parameter problem, in heavily stocked tanks, or as part of a deep clean. Changing more than 50% at once is riskier and should be reserved for genuine emergencies (an ammonia spike in an uncycled tank, contamination, a severe algae bloom). The bigger the change, the more important it is to match the new water's temperature and parameters exactly.
What you need
- A gravel vacuum (siphon) sized to your tank. A 100 L tank wants a medium siphon, not a tiny one; undersized siphons double the time. The Python-style no-spill water changers (sold under several brand names) connect directly to a tap and remove the bucket entirely, and are worth the investment once you are past 100 L.
- A clean bucket used only for the aquarium. Soap residue from a household bucket can kill fish, so keep a dedicated one. A 10 to 15 L bucket suits most tanks; smaller means more trips.
- Dechlorinator, covered in detail below. Seachem Prime is the most concentrated and widely used; API Tap Water Conditioner works fine too. Both are stocked in every Australian aquarium shop.
- A thermometer to match temperatures. Anything accurate to within a degree is enough.
- An algae scraper or magnet for cleaning the glass while the water is partly drained (much easier than scrubbing through a full tank).
How to do a water change, step by step
Before you start
You can leave the filter running for small water changes. Most filters keep working with the intake submerged even when the water level is 25% lower. For larger changes (50% or more), where the filter intake would be exposed, unplug it too.
Remove the old water
Start the siphon (most gravel vacs prime by submerging or squeezing the bulb) and run it through your substrate as you drain. The gravel vacuum lifts and tumbles the substrate, pulling out trapped waste and uneaten food while leaving the substrate behind. Move methodically across the tank floor, focusing on areas with visible debris.
If your tank is planted, vacuum only the open areas and the surface above the plants. Do not disturb the planted root zones, where beneficial bacteria and nutrients build up.
Stop when you have removed about 25% of the water (or your target amount). For most tanks that means draining until the water line drops a few centimetres.
Clean while the water is low
This is the right moment to scrape algae off the glass, prune plants, and remove any obvious debris. Everything is easier with less water in the way, and any disturbed muck goes out with the next siphon pass.
Prepare the new water
Fill your bucket (or new-water vessel) with tap water at roughly the same temperature as your tank; check with a thermometer rather than guessing. Add dechlorinator at the dose on the label, dosing for the volume you are adding (not your full tank); our water conditioner calculator works out the millilitres for Seachem Prime, API, or any other brand. Stir briefly.
If you use a Python-style direct-to-tap changer, you can add dechlorinator to the tank itself before refilling. Most products explicitly support this; check your bottle's instructions.
Add the new water
Pour or pump the new water back in slowly. Pouring straight onto delicate plants can damage them, and a forceful refill stirs up the substrate and re-clouds the water. Aim the flow against the glass or onto a flat decoration to break its force.
Restart equipment
Wait a minute for the water to settle, then plug the heater back in. If you turned the filter off, restart it now and check that flow returns to normal within a few seconds. If it does not, the impeller may need a quick prime (instructions vary by filter).
Confirm the temperature with your thermometer one more time. If it has drifted more than a degree or two from the target, let the heater bring it back before calling the change finished.
Why dechlorinator is non-negotiable
Municipal tap water in Australia is treated with chlorine or chloramine to kill bacteria, including, unfortunately, the beneficial bacteria living in your filter and on every surface of your tank. Chlorine also damages fish gills, especially during the direct contact as new water hits them.
Dechlorinator (sometimes called a "water conditioner") neutralises both chlorine and chloramine within seconds, making tap water safe for fish. It is not optional, and you cannot substitute it by letting water "stand overnight": chlorine evaporates given enough time, but chloramine does not, and most Australian supplies now use chloramine specifically because of this.
A bottle of Seachem Prime treats thousands of litres at the standard dose and lasts most keepers a year or more. The cost per water change is negligible. There is no reasonable scenario where skipping dechlorinator makes sense.
Matching temperature and other parameters
Sudden temperature changes are one of the most common causes of stress after a water change. The general rule is to keep the new water within 2°C of your tank's temperature. For most setups that means mixing hot and cold tap water to roughly match, then checking with a thermometer before adding dechlorinator.
For most community tanks, temperature is the only parameter you actively need to manage on a routine change. Your source water's pH, hardness, and mineral content are stable enough between changes that small variations do not significantly stress fish.
Exceptions where parameters matter more:
- Soft-water species (neon tetras, discus, certain shrimp) in hard tap water: you may want to remineralise with RO water or use a peat-based softener. Worth discussing with an experienced keeper of your specific species.
- Hard-water species in soft tap water: guppies, platies, livebearers, and most snails do poorly in very soft water, so supplemental hardness may be needed.
- Very large changes (50% or more): even small parameter differences become significant at high replacement percentages. Match more carefully.
If you have unusually variable tap water (well water, or water that changes with the seasons), keeping a small log of pH and hardness readings can flag drift before it becomes a problem.
Common mistakes that hurt fish
Skipping dechlorinator "just this once"
A single dechlorinator-free water change can wipe out a big portion of your beneficial bacteria, triggering an ammonia spike days later. The fish you saved the price of dechlorinator on will pay the cost.
Cleaning the filter media at the same time
The filter media holds the bulk of your beneficial bacteria. Cleaning it in tap water during a water change is a one-two punch that can crash a cycle. If your filter genuinely needs maintenance, do it on a different day, and rinse the media only in the old tank water you removed (in a clean bucket), never under the tap.
Stirring up planted root zones
The substrate around plant roots holds nutrients and beneficial bacteria. Aggressive vacuuming there removes both and damages the roots. Vacuum the open areas and leave the planted zones alone.
Refilling too fast
A forceful refill stirs up the substrate, clouds the water for hours, and stresses fish. Pouring slowly against the glass takes thirty extra seconds and prevents both.
Adding cold water to a tropical tank
"Close enough" is not close enough for the fish. A 5°C drop during a water change can trigger ich outbreaks, weaken the immune system, and cause visible stress. Use a thermometer.
Doing very large changes "just to be safe"
Routine 75% or 100% water changes do not help; they shock the fish and disrupt parameter stability. Bigger is not better. Consistency is better.
When the usual rules don't apply
Heavily planted tanks
Densely planted tanks with light to moderate stocking use up much of the nitrate that water changes would normally export. Some advanced planted-tank keepers genuinely change water only every 2 to 3 weeks. The signal is your nitrate reading: if it stays comfortably under 20 ppm, you can stretch the schedule. Do not jump from "I have plants" to "I can skip water changes"; the maths has to actually work.
Shrimp-only and breeding tanks
Shrimp colonies (cherry shrimp, crystal shrimp) are especially sensitive to parameter swings. Smaller, more frequent changes (10 to 15% twice a week) keep parameters steadier than large weekly ones. The goal is for no single change to shift parameters meaningfully.
Brand-new tanks (first 8 weeks)
A newly stocked tank is still settling biologically. Stick to a rigid weekly 25% schedule for the first 8 weeks regardless of test readings, then adjust based on observed nitrate.
After medicating
Many medications need a series of large water changes after treatment to remove leftover medication. Follow the specific instructions on the product; a generic "weekly 25%" does not apply during and immediately after treatment.
After an emergency (ammonia spike, contamination)
Large emergency water changes (50 to 75%) are appropriate when something has gone wrong. Use dechlorinator generously, match the temperature carefully, and repeat daily until parameters are safe.
Troubleshooting
My fish look stressed after every water change
Almost always one of three causes: a temperature mismatch, skipped or underdosed dechlorinator, or too large a change. Check each in order. If the problem persists, test your tap water itself; some supplies have variable chloramine levels, and very occasionally elevated copper from old pipes.
My nitrate is still high even with weekly 25% changes
Increase to 40 to 50% weekly, or 25% twice a week. If your tap water itself has nitrate (common in some agricultural areas), no amount of changing will get you below the source level. You may need to reduce stocking or add more plants.
My water gets cloudy after every change
You are either disturbing the substrate during the refill (slow the flow down) or your filter is being briefly overwhelmed by lifted detritus. Both clear within hours and are not dangerous.
I can't keep up with weekly changes. Is fortnightly okay?
For light stocking with plants, yes, as long as nitrate stays under 20 ppm. For typical stocking, fortnightly will gradually drift your parameters in the wrong direction, with more algae and slow-building problems. Better to do a smaller weekly change than a larger fortnightly one.
Building a sustainable habit
The hardest part of water changes is not the technique; it is the consistency. Three things genuinely help:
Pick a fixed day and time. Sunday morning, Wednesday evening, whatever works. The keepers who do weekly water changes for years are the ones who do not have to decide each week.
Keep your equipment ready. A siphon, bucket, and dechlorinator stored together within reach of the tank removes a small barrier each time. The keepers who skip water changes are usually the ones whose gear is buried in a cupboard.
Make it small enough to actually do. A 25% change on a 100 L tank is about 25 litres, or fifteen minutes including setup and cleanup. If your routine takes much longer than that, your setup is over-engineered. Streamline it.
The reward for consistency is real: tanks with regular water changes have healthier fish, fewer algae problems, more stable parameters, and far fewer of the slow declines that frustrate keepers. The work compounds.
For complete tank setup from scratch, see how to set up a new aquarium. For establishing the biological filter, see cycling guide. For understanding what to test, see water parameters explained.