Changing the water in a fish tank is something every fish owner needs to do to keep their fish healthy. Even when the water looks clean, waste from fish and leftover food slowly builds up, which can cause problems if it is not dealt with properly. This is why learning how to change fish tank water the right way is so important.
Changing fish tank water does not mean emptying the whole tank. It simply involves removing some of the old water and replacing it with fresh, treated water. When done correctly, this keeps the water balanced, reduces stress for the fish, and helps the tank stay healthy over time. This guide explains how to change fish tank water in a clear and easy way.
How Much Water to Change Each Time
The amount of water you change is just as important as how often you do it.
Safe Water Change Percentages
| Situation | Water to Change |
| Routine maintenance | 10–20 percent |
| Fortnightly change | 20–25 percent |
| High nitrate levels | Up to 30 percent |
| Emergency only | Up to 50 percent |
Avoid full water changes unless there is a serious emergency. Removing too much water at once disrupts the nitrogen cycle and can shock fish.
If you are unsure, it is always safer to change less water more often than large amounts occasionally.
Tools You Need to Change Fish Tank Water
You do not need complicated equipment, but using the right tools makes the process easier and safer.
Essential Equipment
| Tool | Purpose |
| Siphon or gravel vacuum | Removes water and waste |
| Clean bucket | Holds old and new water |
| Water conditioner | Removes chlorine and chloramine |
| Thermometer | Matches water temperature |
| Towels | Catch spills and splashes |
Buckets should be dedicated to aquarium use only. Never use buckets that have held cleaning products or chemicals.
Preparing New Water Before You Start
New water must be safe before it enters the tank. Tap water contains chlorine and sometimes chloramine, both of which are harmful to fish and beneficial bacteria.
Preparing Water Correctly
- Fill a clean bucket with tap water
- Add the correct dose of water conditioner
- Allow the water to reach room temperature
- Check temperature against the tank
Some people allow water to stand for several hours, but the conditioner works immediately, making the water safe straight away.
Preparing water before you begin avoids rushing and reduces stress during the water change.
Matching Water Temperature Correctly
Temperature shock is one of the biggest risks during a water change. Even a small difference can stress fish.
Temperature Matching Tips
- Use a thermometer, not your hand
- Aim for the same temperature as the tank
- A difference of more than 1–2 degrees can cause stress
- Adjust by mixing hot and cold tap water slowly
Fish are sensitive to sudden changes. Taking a few minutes to match temperature properly can prevent long-term problems.
Turning Off Equipment Safely
Before removing water, always turn off electrical equipment.
What to Switch Off
- Heater
- Filter
- Air pump (if fitted)
Heaters can crack if exposed to air while hot. Filters can run dry and become damaged. Turning everything off protects both your equipment and your fish.
How to Remove Old Tank Water Using a Siphon
This is the core step in changing fish tank water.
Step-by-Step Siphoning
- Place one end of the siphon into the tank
- Position the other end in a bucket below tank level
- Start the siphon according to the manufacturer’s design
- Move the siphon slowly over the substrate
- Stop once the correct amount of water is removed
Move calmly and steadily. Fish quickly get used to the process if it is done gently.
Removing Waste While Changing Water
While siphoning, you will remove loose waste from the gravel or sand. This is not deep cleaning, but it is an essential part of keeping your fish tank clean. Removing debris during a water change helps prevent waste buildup and ensures a healthy tank environment.
What to Focus On:
- Uneaten food
- Visible debris
- Light surface waste
Avoid disturbing deep layers of substrate, especially in mature tanks, where beneficial bacteria live and should not be removed. Regularly cleaning your fish tank in combination with water changes ensures the best environment for your fish.
Adding Fresh Water Back Into the Tank Safely
Refilling the tank should be slow and controlled.
Best Refill Practices
- Pour water gently down the side of the tank
- Avoid pouring directly onto fish or substrate
- Use a jug if the bucket is heavy
- Pause if fish appear startled
Slow refilling prevents cloudy water and reduces stress.
Restarting Filters and Heaters After the Water Change
Once the tank is full:
- Ensure the heater is fully submerged
- Switch the heater on
- Restart the filter
- Check water flow and temperature
Leave the tank lights off for a short while to help fish settle.
What to Do If Fish Act Oddly After a Water Change
Some fish behave differently after a water change. Mild changes are normal.
Normal Behaviour
- Temporary hiding
- Increased swimming
- Mild curiosity
Signs to Watch For
| Behaviour | Possible Cause |
| Gasping at surface | Temperature or oxygen issue |
| Darting | Stress or shock |
| Lethargy | Water chemistry change |
If behaviour continues for more than a few hours, test water parameters and check temperature again.
How Often You Should Change Fish Tank Water
There is no single schedule that fits every tank, but most aquariums fall into predictable patterns.
General Guidelines
| Tank Type | Recommended Frequency |
| Small tanks under 60 litres | Weekly |
| Medium tanks 60–150 litres | Weekly or fortnightly |
| Large tanks over 150 litres | Fortnightly |
| Heavily stocked tanks | Weekly |
| Lightly stocked tanks | Fortnightly |
Note:
- New tanks often need more frequent changes while the biological cycle stabilises.
- Established tanks with stable readings may cope well with slightly longer intervals.
Consistency matters more than perfection. A regular routine keeps water conditions stable and predictable for fish.
Learning how to change fish tank water properly is one of the most valuable skills in fish keeping. It protects fish health, stabilises water conditions, and prevents many common problems before they start.
To help maintain a healthy environment for your fish, having the right fish tank is essential. Explore our range of high-quality fish tanks that are perfect for all types of fishkeeping. Whether you’re a beginner or an experienced owner, we have the perfect tank for your needs.
FAQs
1. Can I change all the water in my fish tank at once?
Changing all the water at once is not recommended as it can disrupt the tank’s natural balance and harm your fish. It may also kill the beneficial bacteria that help maintain water quality. Instead, perform partial water changes of 10-30% depending on your tank size and water quality.
2. How do I know when it’s time to change the water in my fish tank?
If your fish seem stressed, water appears cloudy, or there’s a strong smell, it’s time to change the water. Regularly testing the water for nitrate, nitrite, and ammonia levels can also help you determine when a water change is needed.
3. How do I choose the right fish tank size for my home?
The ideal fish tank size depends on the space available and the type of fish you plan to keep. Larger tanks are generally better for fish health, as they provide more stable water conditions.
4. How do I create a natural environment for my fish?
To create a natural environment, use live plants, natural rocks, and proper substrate. Ensure the water is properly conditioned, and maintain correct temperature and pH. Fish enjoy hiding spots, so provide caves or plants where they can seek shelter, mimicking their natural habitat.