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 3 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 doesn't 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 from your water (calcium, magnesium, carbonate hardness), and a slow shift in pH as the water's buffering capacity erodes.

The solution isn't a filter, a chemical, or a plant — it's replacing some of the old water with new water. A water change exports built-up nitrate and waste, replenishes the minerals your fish and plants need, and resets your tank's chemistry toward what's coming out of the tap. There is no equipment that substitutes for this. Skipping water changes is the single most common reason long-running tanks decline in health.

This isn't optional or "for beginners." Every serious fishkeeper does water changes regularly, including in heavily planted tanks, including in low-stocked tanks, including in tanks that "look fine." A tank can look fine and still be slowly poisoning its inhabitants with accumulated nitrate.

How often you should change water

The honest answer is that frequency depends on your stocking, your plant load, your filtration, and your 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 (typically under 20 ppm), to prevent dissolved organics from accumulating to a point where they affect fish, and to keep your water parameters stable. It's also infrequent enough not to stress you or your fish.

Some situations warrant adjustment:

• Heavily stocked tanks may need water changes twice a week, or larger weekly changes (40–50% rather than 25%).
• Heavily planted tanks with light stocking often need less — every 10–14 days can be enough, because plants consume nitrate directly.
• Shrimp-only and breeding tanks typically benefit from smaller, more frequent changes (10–15% twice a week) to maintain very stable parameters.
• New tanks in their first few months are more fragile and benefit from rigid weekly schedules even if testing suggests less is needed.

The signal that you're doing too few water changes is rising nitrate. If your nitrate reading stays under 20 ppm with weekly 25% changes, your schedule is fine. If it creeps above 40 ppm between changes, increase frequency or percentage.

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's enough to meaningfully reduce nitrate (a 25% change removes roughly 25% of dissolved waste), but small enough to avoid shocking your fish with sudden parameter shifts.

To work out 25%, you need to know your tank's actual water volume — which is not the same as the tank's stated capacity. A "100-litre" aquarium typically holds around 85–90 litres of water once you account for substrate, decor, and the gap between the water line and the rim. Most keepers learn their tank's real volume by simply 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. Larger than 50% in a single change is risky and should be reserved for genuine emergencies (ammonia spike in an uncycled tank, contamination, 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 100L tank wants a medium-sized siphon, not a tiny one — undersized siphons make water changes take twice as long. The "Python" no-spill water changers (sold under several brand names) hook directly to a tap and remove the bucket entirely; worth the investment once you're past 100L.
• A clean bucket used only for the aquarium. Soap residue from household buckets can kill fish — keep a dedicated one. A 10–15L bucket is the right size for most tanks; smaller means more trips.
• Dechlorinator — covered in detail below. Seachem Prime is the most concentrated and widely-used option; API Tap Water Conditioner works fine; both are stocked in every Australian aquarium shop.
• A thermometer to match temperatures. Anything that reads accurately to ±1°C is enough.
• An algae scraper or magnet for glass cleaning while the water is partially drained (much easier than scrubbing through full water).

How to do a water change, step by step

Before you start

Unplug your heater. Heaters left running in air can crack from thermal stress or, worse, overheat the small remaining water if the water level drops below them. Unplugging the heater is one of the most-skipped steps and one of the most damaging.

You can leave the filter running for short water changes — most filters keep working with the intake submerged even when the water level is 25% lower. For larger changes (50%+) where the filter intake would be exposed, unplug it too.

Remove the old water

Start the siphon (most gravel vacs prime by submerging or by squeezing the bulb), and run it through your substrate as you drain. The gravel vacuum lifts and tumbles the substrate, removing trapped waste and uneaten food while leaving the substrate itself behind. Move methodically across the tank floor, focusing on areas with visible debris.

If your tank is planted, vacuum only the open substrate areas and the surface above plants — don't disturb planted root zones, where beneficial bacteria and nutrients build up.

Stop when you've removed about 25% of the water (or your target percentage). For most tanks this 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 the 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 recommended on your product's label, dosing for the volume you're adding (not your full tank volume). Stir briefly.

If you're using a Python-style direct-to-tap changer, dechlorinator can be added 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 into the tank slowly. Pouring straight onto sensitive plants can damage them, and a forceful refill can disturb the substrate and re-cloud the water. Aim the flow against the glass or onto a flat decoration to break the 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 not, the impeller may need a quick prime (instructions vary by filter).

Confirm the water temperature with your thermometer one more time. If it's drifted more than a degree or two from the tank's target, allow the heater to bring it back up before considering the change finished.

Why dechlorinator is non-negotiable

Municipal tap water in Australia is treated with either 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, particularly during direct exposure when new water hits them.

Dechlorinator (sometimes called a "water conditioner") neutralises both chlorine and chloramine in seconds, making tap water safe for fish. It is not optional, and it is not something you can substitute by letting water "stand overnight" — chlorine evaporates given enough time, but chloramine doesn't, and most Australian municipal 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 in which skipping dechlorinator makes sense.

Matching temperature and other parameters

Sudden temperature changes are one of the most common causes of fish stress after a water change. The general rule is to keep the new water within ±2°C of your tank's temperature. For most freshwater setups this means mixing hot and cold tap water to roughly match, then verifying with a thermometer before adding dechlorinator.

For most community tanks, matching 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 don't 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; supplemental hardness may be needed.
• Very large changes (50%+) — 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 seasonally), 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 significant portion of your beneficial bacteria, triggering an ammonia spike days later. The fish you're saving the price of dechlorinator for 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 media only in the old tank water you've removed (in a clean bucket), never under the tap.

Stirring up planted root zones

Substrate around plant roots accumulates nutrients and beneficial bacteria. Aggressive vacuuming there removes both and damages the root system. Vacuum the open areas; 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, immune suppression, and visible stress. Use a thermometer.

Doing very large changes "just to be safe"

Routine 75% or 100% water changes don't help; they shock the fish and disrupt parameter stability. Bigger isn't better; consistency is better.

When the usual rules don't apply

Heavily planted tanks

Densely planted tanks with light to moderate stocking consume much of the nitrate that water changes would normally export. Some advanced planted-tank keepers genuinely change water only every 2–3 weeks. The signal is your nitrate reading — if it stays under 20 ppm comfortably, you can stretch your schedule. Don't extrapolate from "I have plants" to "I can skip water changes"; the math has to actually work.

Shrimp-only and breeding tanks

Shrimp colonies (cherry shrimp, crystal shrimp) are particularly sensitive to parameter swings. Smaller, more frequent changes — 10–15% twice a week — keep parameters more stable than large weekly changes. The goal is to never have any single change 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 require a series of large water changes after treatment to remove residual medication. Follow the specific instructions on the product — generic "weekly 25%" doesn't apply during and immediately after treatment.

After an emergency (ammonia spike, contamination)

Large emergency water changes (50–75%) are appropriate when something has gone wrong. Use dechlorinator generously, match temperature carefully, and repeat daily until parameters are safe.

Troubleshooting

My fish look stressed after every water change

Almost always one of three causes: 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 municipal 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 40–50% weekly, or 25% twice a week. If your tap water itself has detectable nitrate (common in some agricultural areas), no amount of changing will get below the source level — you may need to reduce stocking or add more plants.

My water gets cloudy after every change

You're either disturbing the substrate during refill (slow down the flow), or your filter is being briefly overwhelmed by lifted detritus. Both resolve within hours and aren't 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 parameters in the wrong direction; expect more algae and slower-growing problems. Better to do a smaller weekly change than a larger fortnightly one.

Building a sustainable habit

The hardest part of water changes isn't the technique — it's the consistency. Three things that 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 don't 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 equipment is buried in a cupboard.

Make it small enough to actually do. A 25% change on a 100L tank is roughly 25L — fifteen minutes including setup and cleanup. If your routine takes longer than that, your setup is over-engineered. Streamline.

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.