The 30-second verdict
If you do one thing for your tank, do this. A regular water change is the closest thing the hobby has to a cure-all: it removes nitrate, replaces trace minerals, dilutes anything invisible that is building up, and resets the tank toward stable. Most "my fish keep getting sick" problems are really "I don't change enough water" problems.
The routine:
- How much: 20–30% of the tank volume.
- How often: once a week for most tanks.
- The two rules that matter most: dechlorinate the new water, and match its temperature. Get either wrong and a water change becomes the thing that kills the fish.
It takes about 15 minutes once you have a rhythm. Here is the whole thing.
What you need
- A gravel siphon (vacuum) with a hose — the single most useful maintenance tool there is.
- A bucket kept only for the aquarium — never one that has held soap or cleaning product. Label it. Traces of detergent kill fish.
- Dechlorinator (water conditioner) rated for chlorine and chloramine.
- A thermometer.
The step-by-step
- Turn off the heater and filter. A heater exposed to air as the level drops can crack or burn out; a filter running dry can be damaged. Switch both off at the wall before you start.
- Siphon out 20–30% of the water using the gravel vacuum. Push the wide end into the substrate as you go — this pulls out the trapped fish waste and uneaten food that is the real source of your nitrate. Work across different areas of the gravel each week rather than digging the whole bed at once.
- Stop at a quarter to a third down. There is no need to remove more, and a bigger change is a bigger shock.
- Prepare the new water in the clean bucket. Fill with tap water, add dechlorinator at the dose on the bottle, and — this is the step people skip — check the temperature against the tank with a thermometer. It should match within a degree or two.
- Pour the new water in gently. Pour onto a rock, a plate, or your hand to avoid blasting a crater in the substrate and uprooting plants.
- Turn the heater and filter back on. Check both are actually running before you walk away.
That is the entire procedure. Everything below is the why, and the two ways it goes wrong.
The dechlorinator rule
Tap water is disinfected with chlorine or chloramine. Both are lethal to the bacteria colony in your filter — the colony that is the only thing between your fish and their own ammonia. Kill it with untreated tap water and you cause an ammonia spike a few days later that looks like a mystery illness.
Dechlorinate every drop of new water, every time. Including "just a small top-up."
And the local twist: you cannot rely on "leaving water out overnight" here. That old trick removes free chlorine but does nothing to chloramine, which many supplies use — and you cannot tell which you have this week. Use a conditioner that explicitly neutralises both. The full explanation: Is Dubai tap water safe for fish?
The temperature rule — and why it is worse in the UAE
New water that is much colder or hotter than the tank shocks the fish. Temperature shock suppresses their immune system and is a classic trigger for ich, the white-spot parasite. It kills faster than a stable "wrong" temperature ever would.
In most of the world this means "don't use cold tap water in winter." Here, the danger runs the other way and it is specific to this country:
In a UAE summer, the "cold" tap is not cold. Buildings with rooftop or exposed water tanks deliver water at 35 °C or beyond in the afternoon — hotter than your tank. Pour that into a 27 °C tank without checking and you have shocked every fish in it.
So always check the bucket temperature with a thermometer, and if it is off, let it stand to settle or mix to match. Do water changes in the early morning when the building's tank is coolest. And never correct the temperature with hot tap water — the hot line can carry dissolved copper from the plumbing, which is lethal to shrimp and hard on fish. Boil kettle water to mix if you must.
Top-up vs water change: not the same job
This distinction quietly ruins UAE tanks over a summer, so it belongs in every maintenance article.
- Evaporation removes water and leaves the minerals behind. The tank gets steadily harder and more concentrated. You top this up with RO or distilled water, because you are only replacing what evaporated.
- A water change removes water and the dissolved waste in it, and you replace both with dechlorinated tap water.
In this climate evaporation is fast, so a tank that is only ever topped up — never changed — drifts harder and accumulates nitrate invisibly. Do both jobs, with the right water for each. (The full weekly rhythm here.)
How much, how often
| Situation | Change | Frequency |
|---|---|---|
| Standard community tank | 20–30% | Weekly |
| Heavily stocked / cichlids / goldfish | 30–40% | Weekly, sometimes twice |
| Shrimp tank | 10–20% — small and slow | Weekly. Big sudden changes trigger fatal moults |
| New / cycling tank with fish | Whatever keeps ammonia + nitrite near 0 | Daily if needed |
| After a power cut or big filter clean | 25%+ | And test daily for a week |
Let the nitrate decide. If your nitrate is creeping up week to week, change more or more often. If it stays low, your routine is working. This is exactly what the test kit is for: reading your numbers.
On very large changes: a 50%+ change is an emergency tool (an ammonia spike, a pollutant), not routine. The bigger the swing in one go, the more it stresses the fish — which is why little-and-often beats big-and-rare.
What not to do
| Don't | Why |
|---|---|
| Change 100% of the water | Strips the tank of its stable chemistry and beneficial bacteria in the water column. Never necessary; almost always harmful. |
| Rinse the filter media in tap water at the same time | Chlorine kills the bacteria. If the filter needs cleaning, rinse it gently in the old tank water you just siphoned out — and ideally not the same day as a big water change. Why. |
| Forget the dechlorinator "just this once" | That once is enough to damage the colony. Make it automatic. |
| Use a bucket that held soap | Detergent residue is lethal and impossible to fully rinse. Dedicate a bucket. |
| Pour water straight onto the substrate | Craters the bed, uproots plants, clouds the tank. Pour onto a plate or your hand. |
| Do it in the hot afternoon in summer | The tap water is at its hottest and the shock risk is highest. Early morning is safest. |
Making it a habit
The reason people skip water changes is friction — an awkward tank position, a buried filter, a bucket that lives in another room. Reduce the friction and you will actually do it:
- Keep the kit together — siphon, bucket, conditioner — near the tank.
- Same day each week. Sunday-evening tank, whatever fits. A rhythm you keep beats a perfect routine you skip.
- Position the tank for access from the start — room for a bucket underneath, reachable back and top. A tank that is a fight to maintain is a tank that gets neglected: placement.
Common questions
Can I use a python / hose that drains to the sink instead of buckets? Yes, and for larger tanks it is far easier — but you lose the separate step of temperature-matching and dechlorinating in a bucket, so you must dose conditioner for the whole tank volume as it refills and watch the tap temperature carefully. In a UAE summer that tap-temperature caveat is not optional.
Do I need to remove the fish? No. Fish stay in during a normal water change — netting them out is far more stressful than a careful 25% change.
My tank is planted — do I still gravel-vacuum? Lightly, over open areas. In a densely planted tank the roots and substrate are a system you don't want to tear up; just skim detritus off the surface and rely on the plants to use the nutrients.
The water goes cloudy after a change. Usually disturbed substrate (settles in hours) or a bacterial bloom in a young tank. If it keeps happening, you are digging the gravel too aggressively. More on cloudy water.
What is the one thing to never skip? Dechlorinator, temperature-matched. Those two are the difference between the most beneficial thing you do for the tank and the thing that kills it.