How to change the water in a fish tank – Why it’s important!

When cleaning your fish tank one of the most important part is to remove about 15-20% of the water and clean out all the debris from within the gravel and tank. How to change the water in a fish tank and why it’s important is a fundamental part of fish keeping.

In this article, we’ll explain why and how you should safely change the water in your fish tank and what could go wrong if you don’t

The easiest way to do this is with a fish tank siphon or gravel cleaner!

In this article we will also review the Best siphons and gravel cleaners for a fish tank, we explain how and why you should use one.

Why you need to clean your tank.

If you look at your fish tank you might be fooled into thinking that it looks fine. Your water isn’t cloudy and there doesn’t seem to be a lot of dirt in the tank.

So why do you need to clean it?

Simply stir the gravel up a bit and you’ll soon find out why. You will be surprised how much dirt and debris is trapped in the gravel. There is also a build-up of urine and feces that you can’t see.

But the number one reason for changing the water in your fish tank is to reduce the build-up of ammonia in your tank.

The bacteria in your filter should convert ammonia firstly into nitrite and then into nitrate. While ammonia and nitrite are highly toxic to fish, nitrate is much less so. However, as nitrate build up, fish will eventually be affected. So removing dirt and waste will slow down the build-up of both.

Read all about ‘ The Nitrate Cycle

The nitrate cycle in a fish tank

How can you detect the build-up of Nitrates?

You cannot see Nitrates and phosphates building up in your tank with the naked human eye. There are, however, tell tail signs. The build-up of algae is one obvious one. If you’ve never had issues with algae then suddenly it starts to grow quickly in your tank, that’s a warning sign.

Another is an outbreak of a disease in your tank. Stress is the number one factor for fish getting ill and high nitrate levels will cause your fish stress and in return, they will be more susceptible to disease.

Many fish will not show signs of high Nitrate levels until they reach a critical point and then it can be too late. The perfect level for a Freshwater tank is between 5-10ppm ( Parts per Million ) any level between 25-50 would be a concern. Some fish won’t show any signs of effect until the Nitrate levels reach 0ver 100ppm.

Studies have shown that prolonged exposure to very high nitrate levels can affect the growth of young fish, and decrease the likelihood of them reproducing.

Another telltale sign is your fish becoming lethargic and visible sores and blotches on their skin and in general just poor health within your tank. All these are signs of poor water quality and the build-up of Ammonia, nitrates, and Phosphates. In high extreme levels not of these are good for your fish.

how to do a water change
Amazing transformation after cleaning!

The solution for poor fish tank water quality

There are a number of ways to improve the water quality in your fish tank, below is a selection:

  • Weekly water changes
  • Stop overfeeding
  • Overstocking of fish
  • Dirty Filters
  • Poor water circulation
  • Remove decaying organic material
  • Add more plants to eat up the nitrates

The best way to control the quality of your water is to perform weekly water changes. Small and often water changes will guarantee the best quality of water for your fish.

It’s not just the simple fact of replacing dirty water with clean, or removing the nasty things in the water like ammonia, nitrates, and phosphates. There are other elements in the water to consider.

Trace elements and minerals in the water are also vital for the health and well-being of your fish. Over time the natural minerals they are used up or filtered out. If you don’t replace them, the pH of the water will drop. Furthermore, the lack of trace minerals will have a negative effect on the health of your fish.

Giving your fish freshwater regularly is similar to giving your Children vitamins to keep them strong and healthy. Or another analogy is: Imagine sitting in a room full of smoke all day long, breathing in all those nasty chemicals and poisons in the air will affect you long term. Well, fish cannot leave the room or escape this.

It’s up to us as responsible fish keepers to make regular water changes and ensure the best possible environment for our fish to live in.

How to do a water change
Gravel cleaners are amazing! Best purchase ever.

How to change the water in a fish tank

Step by step guide to performing a partial water change in your Freshwater fish tank.

Equipment needed:

  1. Siphon or Gravel cleaner
  2. Plastic bucket x 2
  3. Tap water Dechlorinator
  4. Thermometer
  5. A small plastic plate

Partial weekly water changes will ensure you give your fish the best possible water conditions to thrive in. Follow these simple step by step instructions.

  • Fill a plastic bucket with water and add a Dechlorinating solution to ensure the tap water is safe.
  • Bring the temperature in the bucket to a similar temperature to your tank. You can do this by simply adding hot water and monitoring the temperature using a digital thermometer. Alternatively, you can use a spare fish tank heater set at the correct temperature.
  • Switch of the lighting and heating supply for the fish tank to ensure safety.
  • Using your gravel cleaner start to slowly siphon off the fish tank water and clean the gravel at the same time by pushing the gravel cleaner into the gravel/sand base of your aquarium.
  • Use the tap on the siphon to ensure a steady but slow flow of water is coming out. This will give you more time to work the dirt from the gravel bed.
  • Remove 25% maximum of the old water.
  • Whilst the water level is low, use this time to clean the algae from the tank glass. You can use a magnetic algae cleaner or a new chemical-free cloth and manually do it.
  • Once you have removed 25% of the water and cleaned the glass you can start to fill the tank back up again.
  • Place a small plastic plate on the surface of the water and make sure it floats. Then slowly pour the new water at the correct temperature back into the tank using the plastic plate to allow the new water to enter the tank with minimal disturbance to the fish and plants.
  • Never overfill the tank, allow 2″ from the top of the tank for splashes and evaporation.
  • Once filled to the correct level, plug back in the heater and lighting.
  • Allow the tank to balance out in temperature and calm down before feeding your fish.

The idea is to do a partial water change weekly with minimal disturbance to your fish to avoid stress.

Final thoughts

Doing a water change each week in your fish tank is probably the most boring part of fish keeping, but if you’re going to keep fish in your home then you should act in a responsible manner and look after them the best you can.

You spend all that money and time and effort setting up the tank so why not look after it correctly.

Adding certain fish like Siamese Algae Eaters will help clean up some of that unwanted green algae, but don’t rely on them alone to do the job. They need fresh clean water like any other fish.

We hope you enjoyed and found this article ‘ How to change the water in a fish tank ‘ helpful? If so please share and comment below. Let us know how you change the water in your fish tank.


We wish you Happy Fishkeeping!

Carl Broadbent