How to Do a Water Change in an Aquarium

How to Do a Water Change in an Aquarium

Water changes are the unglamorous part of fishkeeping, but they fix more problems than any bottle on the shelf. Cloudy water, algae kicking up, fish acting off, mystery deaths after you "cleaned" the tank - a steady water change routine prevents a lot of that.

This guide is how I actually do water changes at home: quick, repeatable, and gentle on the fish. I'll walk you through how much to change, how often, what gear makes it easier, and the little details that keep you from swinging pH, temperature, or stressing the whole tank.

What a water change does (and what it does not)

A water change removes dissolved waste (like nitrate and other organics), refreshes minerals, and gets rid of the stuff you cannot net out. Think of it as diluting the bad and topping up the good.

It does not replace your biological filter. Your bacteria live mostly on surfaces (filter media, substrate, rocks), not floating in the water column. So you are not "resetting" the cycle by changing water, unless you do something extreme like draining everything and scrubbing media under hot tap.

How much water to change

Most community tanks do great with 20-30% at a time. That amount moves the needle on nitrate without yanking the water chemistry around. If you are behind on maintenance, resist the temptation to do a huge swap unless you know your tap water matches the tank pretty closely.

• Lightly stocked, planted tank: 10-20% weekly or every other week (test nitrate and let that guide you)
• Typical community tank: 20-30% weekly
• Messy fish (goldfish, big cichlids, Oscars) or heavy feeding: 30-50% weekly, sometimes split into two changes
• Emergency high nitrate or medication aftermath: 30-50% daily for a few days, watching temperature and dechlorinator

How often to do water changes

Frequency depends on stocking, feeding, plants, and your filter. The easiest way to stop guessing is to test nitrate for a few weeks. If nitrate creeps up week to week, you either need bigger changes, more often, or less waste going in.

• If nitrate stays under ~20 ppm all week: your routine is fine
• If nitrate hits 40+ ppm before your next change: increase volume or frequency
• If ammonia or nitrite shows up: treat it as an emergency and figure out what broke (overfeeding, dead fish, filter issues)

Gear that makes water changes easier

You can do water changes with a bucket and a siphon. But a couple upgrades make it so much easier that you stop procrastinating.

• Gravel vacuum (siphon) sized for your tank (small tube for nano tanks, bigger for 20+ gallons)
• Buckets or a water-change container (dedicated to the aquarium, no soap residue)
• Dechlorinator (water conditioner) that handles chlorine and chloramine
• Thermometer (even a cheap one) to match refill temperature
• Towels and a small sponge for drips
• Optional: Python-style hose system for larger tanks
• Optional: pump or powerhead for refilling (saves your back)
• Optional: algae scraper for quick glass cleanup while water is low

Before you start

A smooth water change is mostly prep. The goal is simple: remove old water and gunk, then replace with dechlorinated water that is close in temperature.

• Turn off heater and filter if the water level will drop below them
• Unplug equipment instead of relying on switches (I have learned this the wet way)
• Have dechlorinator and a thermometer ready
• Decide how much water you are removing (mark the glass with a bit of tape if you like)

Step-by-step water change

1. Unplug heater, filter, and any equipment that might run dry.
2. Start the siphon and drain water into a bucket or to a sink/drain if you have a hose setup.
3. Vacuum the substrate as you drain: push the tube into the gravel, let debris lift, then pinch or lift the tube before you suck up too much gravel. Work in sections, not the whole tank every time if you have deep substrate.
4. Wipe the glass if needed while the water is lower (it is easier to reach and you will not splash as much).
5. Stop draining at your target percentage (20-30% is a great default).
6. Prepare replacement water: match temperature as closely as you can and add dechlorinator.
7. Refill slowly to avoid blasting substrate and stressing fish. Pour onto a plate, bowl, or your hand, or use a gentle pump/hose.
8. Restart filter and heater once the water level is back. Confirm the heater is fully submerged before plugging it in.
9. Check that the filter is flowing normally and the water is clear of big debris. Done.

How to vacuum gravel without making a mess

The trick is to let the gravel tumble inside the vacuum tube. The heavier gravel drops back down, and the lighter mulm gets pulled out. If you jam the tube down and leave it there, you will drain a ton of water from one spot and kick debris everywhere.

• Hover the tube a bit above the surface first to pull loose debris
• Plunge straight down, let the gravel churn for 2-3 seconds, then lift and move on
• In planted tanks, skim around root zones instead of tearing plants up
• In sand, do not plunge deep - hover and lightly stir the top

Dechlorinator and refill water

Tap water almost always contains chlorine or chloramine. Either one can burn gills and damage your filter bacteria. Dechlorinator is non-negotiable if you use tap.

I add dechlorinator based on the amount of new water going in, not the whole tank volume (unless the product label says otherwise). If you are filling directly from the tap with a hose, dose for the full tank volume to be safe, especially with chloramine.

• Bucket method: condition the bucket, then pour it in
• Hose method: dose conditioner into the tank before or during filling, and refill slowly
• Match temperature by feel plus thermometer: close is fine, ice-cold is not

How to avoid shocking your fish

Most fish stress during water changes for three reasons: temperature swings, chemistry swings, and the physical chaos of siphons and splashing.

• Keep refill temperature close (within a couple degrees F/C is my target)
• Do smaller, more frequent changes if your tap water is very different
• Refill slowly and gently so you are not sandblasting the tank
• Leave lights off during and for a bit after if your fish spook easily
• Do not chase fish with the siphon - move calmly and they will usually ignore you

Filter and media during water changes

If you want to keep your tank stable, treat your filter media like it is alive (because it kind of is). I do not replace all media at once, and I never rinse it under chlorinated tap.

• If your filter flow slows down, swish sponges or biomedia in a bucket of old tank water
• Replace mechanical floss only when it is falling apart or you cannot clean it anymore
• If you use carbon, swap it on a schedule only if you actually need it (like removing meds). Otherwise, it is optional
• Do not clean filter and deep-vac the whole substrate on the same day in a small, lightly established tank

Planted tanks and water changes

Plants help, but they do not erase the need for water changes. They do make the tank more forgiving, and they can keep nitrate lower, but you still get dissolved organics and mineral drift over time.

• If you dose fertilizers, water changes help reset imbalances
• Go lighter on gravel vacuuming around heavy root areas
• Trim and remove decaying leaves during the water change so they do not rot in the tank
• If you run CO2, water changes can shift CO2 a bit - watch fish for the first hour after refill

Special cases

Nano tanks

Small tanks swing faster. A two-gallon temperature mismatch in a 5-10 gallon tank is a bigger deal than in a 55.

• Do 10-20% more often instead of big changes
• Preheat refill water in a bucket if the room is chilly
• Refill very slowly (shrimp especially hate turbulence)

Goldfish and messy eaters

Goldfish produce a lot of waste. If you want clear water and healthy fish, plan your routine around them, not around what is convenient.

• 30-50% weekly is common, sometimes twice weekly for crowded setups
• Vacuum more of the bottom each time (they are little waste factories)
• Overfiltering helps, but it does not replace water changes

After medication

Most meds end with a series of water changes to dilute leftovers. This is one place where bigger changes make sense, as long as you keep temperature steady.

• Follow the medication directions first
• Do a 30-50% change, then run fresh carbon if the med label says carbon removes it
• Watch fish closely for a day or two after - some meds irritate gills and they need clean water to recover

Common mistakes I see (and have made)

• Forgetting to dechlorinate (set the conditioner next to your bucket so you cannot miss it)
• Doing a huge water change to fix a long-term neglect problem (do several medium changes instead)
• Leaving the heater on while draining
• Over-vacuuming a new tank and rinsing the filter too aggressively at the same time
• Refilling with very cold water because "it will warm up"
• Cleaning algae and doing a water change, then feeding heavy right after (give the tank a breather)

Quick checklist

• Unplug heater and filter if water level will drop
• Drain 20-30% (or your chosen amount)
• Vacuum substrate lightly while draining
• Condition new water for chlorine/chloramine
• Match temperature closely
• Refill slowly
• Restart filter, then heater
• Check flow and fish behavior for a few minutes

A simple routine that works

If you want a baseline routine that covers most freshwater community tanks: do a 25% water change every week, vacuum a different section of substrate each time, and rinse your filter sponge in old tank water once a month or when flow drops. Test nitrate now and then. If it starts climbing, you adjust the schedule before the tank starts complaining.