How to Change Aquarium Water – Quickly and Safely for Your Fish

The tank your fish live in plays a vital role in their health. And without a doubt, the most important element of any fish tank is the quality of its water.

Imagine being covered in bacteria, pollution and other harmful chemicals, all day every day? Not surprisingly, your health and well-being would suffer. Fish are no different.

This is why it’s important to regularly change an aquariums water, and learn the correct procedure to do so.

How to do a water change written beside a community of goldfish in a deep blue aquarium
© julialine802 / Adobe Stock

Why You MUST Regularly Change Your Fish Tank’s Water

The quality of the water in your tank will determine whether your fish are happy and healthy – or miserable and feeling ill – It’s that vital!

Water that hasn’t been replaced regularly creates an unhealthy ecosystem. Parasites and bacteria thrive in less than acceptable conditions and these are very harmful to fish, often leading to sickness and in some cases, death.

But Doesn’t a Filter Keep The Fish Safe?

We know what you’re thinking, but the answer to this question is unfortunately: ‘No’. Well, the answer is: ‘A filter helps, it’s necessary, but you must still change the water regularly.’

A filter will assist with keeping the chemicals and solid particles in your aquarium water under control – up to a point! However it’s incapable of completely removing the toxins produced and they will build up.

Fish waste turns to ammonia after a short period of time, then bacteria turn the ammonia into other chemicals known as nitrites, before yet more bacteria turn the nitrites into nitrates. This is known as ‘the nitrogen cycle‘.

Both ammonia and nitrites are poisonous to your beloved fish and a filter turns these extremely harmful chemicals into the far less harmful nitrates. However, in high enough concentrations, nitrates still do harm and they can be only be removed through a water change.

So in summary, yes, a filter does keep your fish safe and delays the inevitable. The inevitable being that you must still regularly replace the water.

That’s not All!

A high concentration of goldfish in dark, bubbling water.

Too many Goldfish can result in stunted growth

In goldfish aquariums there is another important reason to frequently change the water: Because goldfish emit a growth inhibiting hormone (or pheromone) which is harmful at high levels.

The theory is that in overcrowded conditions (a high population in a small pond for example) this hormone emitted by all the fish in the water soon reaches high levels and causes the entire population to have stunted growth.

This is a survival mechanism that helps the whole community because if the fish only grow to a smaller size, there’s physically more room for each and they need less food and resources each too.

However, there’s some evidence that their bodies are stunted while their internal organs are not. This leads to internal abnormalities that leads to a premature death.

Studies are inconclusive but some evidence suggests this hormone has other harmful effects towards even fully grown and already matured fish.

Regardless, changing the water regularly removes any doubt so is simply good practice!

Why You Must Follow A Set Procedure

You may well be thinking you can simply siphon out the water, throw some back in and the job is done. Simple right? NO!

The friendly bacteria discussed above that are essential for the water quality in your tank can be very easily killed by not following correct guidelines.

Also, fish are very easily shocked and stressed by any change in water temperature or chemical content.

So water changes not only have to be done – they have to be done right. You will learn that procedure in the rest of this article.

Clean Looking Water – May Not Be Clean!

Remember, just because your tank water might look clean, it doesn’t necessarily mean it is.

The harmful chemicals previously mentioned are invisible to our eyes, we have no way to know just how chemically dirty or toxic tank water is unless we use testing kits.

This is something we advise often on this site, to test your water regularly. But it’s only through regular water changes that you can ever be sure there is a safe and clean ecosystem for your aquatic friends.

How Much Water To Change Each Time? And How Often?

As a general rule of thumb, we recommend a 40% water change, once per week.

We recommend 40% because a change in water condition can shock and stress your fish. The greater the change, the higher the chance of this happening. But the less you replace, the more often you will have to do the job.

A 40% water change is a happy medium to aim for, between changing enough to make a meaningful difference, but not so much that you stress your fish.

However, this 40% rule may also change depending on the following factors:

  • The size of your tank: Smaller ones and bowls (please upgrade if you have a bowl!) either with poor quality or no filtering system will require larger changes more often to keep them in perfect condition.
  • Concentration of fish: Is your tank heavily populated? Remember that the higher the concentration of fish, the more waste and growth hormones being emitted. These are harmful to your fish, so you’ll want to perform a partial water change on a more frequent basis!

The best way to determine how much and how often to change your aquarium water is to test it’s parameters and change in fresh as often as necessary to keep the water quality high.

(We shall write an article on this shortly and link to it from here when done.)

Things to Remember Before You Start

A cartoon fish excited by something read in a book they're holding

Here are a few things to keep in mind that will make the whole process easier, keep your fish safe and make sure your tank stays in optimum condition both during and after the water change:

  • You don’t want to remove the fish when performing your routine weekly water change as it just creates more work for yourself and stresses out the fish!
  • Only occasionally remove any plants and decorations from your tank for cleaning. Beneficial bacteria live on these surfaces and by cleaning or removing them you may kill some of this bacteria.
  • If you do clean any ornaments or plants, do not use household detergents as any traces that make into your tank will cause harm. Only rinse things in old removed water instead.
  • To refill your tank, never use distilled water as it’s just too pure (sounds odd doesn’t it?) and will deprive your fish of important trace elements they require. Use tap water as it contains many important minerals which help to promote excellent health.
  • Only use a bucket which has a sturdy handle and no cracks or holes – You don’t want to create a mess for yourself!
  • Don’t overfill the bucket. Keep them light so they’re easy to lift and carry. Remember you have to lift the bucket higher than the top lip of your aquarium and we don’t want you pulling a muscle or hurting your back.
  • Have some towels handy while performing a change so that you can soak up any spillage. I always place a couple around the edge of my tank cabinets just in case.
  • Slow and steady wins the race. When it comes to filling your tank back up, fast and furious isn’t the way to go, especially with fancy goldfish, weak swimmers or betta splenden who will feel like they’ve been put through a washing machine with the strong currents generated. Add the water in gently and your fish will be thanking you for it.

How to Change Fish Tank Water – A Step-By-Step Guide:

Here are the recommend steps to safely and efficiently change your fish tank water, with minimal fuss and effort while keeping the ecosystem of your aquarium as good as it can be.

First, Make Sure You Have All the Necessary Equipment

Things you’ll need:

  • A sturdy bucket – Clean and free of any contaminants.
  • A gravel vacuum – To remove waste from between your substrate
  • Water conditioner / treatment – To remove chlorine and chloramines
  • A Thermometer – To temperature match new water with old.
  • Very clean and contaminant free hands!

How to Do a Water Change! Step-by-Step

Now we’ve discussed all the why’s and what not’s, finally we can discuss getting down to business.

Don’t feel intimidated, it’s easy to do once you’ve read exactly how:

Removing Old Water and Cleaning any Equipment and Decorations

  1. Ensure that any electronic equipment, such as your filter or air pump, have been switched off.
  2. Using your gravel vacuum, remove both some of the water and waste from your tank. To do this, place one end of your gravel vacuum into the gravel and the other end in your bucket. Start the siphoning action according to your gravel vacuum’s instructions to remove some waste and waste.
  3. While the vacuum is working and water flowing, move the end around in various places in the gravel to remove as much waste as possible. Stop as soon as your bucket is filled.
  4. DON’T pour this water right away though. For each bucket full you remove, use it to clean your filter, any plants, ornaments or equipment you intend to clean. NEVER clean anything from your tank in tap water. It kills the bacteria your tank absolutely needs.
  5. First of all, remove any filter media, add it to a bucket of old tank water then rinse and squeeze out any solid particles from the floss or sponges. This will prevent blockages and a potential loss in filter performance.
  6. Now re-assemble your filter and put it back in place.
  7. If cleaning any plants, ornaments or other equipment (we recommend no more than half at each change to retain as much friendly bacteria living on them as you can), use subsequent buckets of old water to rinse these off before returning to the tank.
  8. Repeat siphoning out one bucket of water at a time until approximately 40% of the total has been removed from the tank. Then you can begin adding new water back in.

Adding Fresh Water Back Into Your Tank

  1. Fill the bucket with fresh tap water, using both hot and cold – together with your thermometer – to temperature match the new water to the temperature in the tank. Remember, sudden temp changes will shock and stress your fish, so this is important!
  2. DON’T add this new water to your tank until you have treated it with a product specifically for removing chlorine, chloramines and ammonia. Tap water contains these chemicals to remove bacteria so it’s safe for us to drink. But these chemicals kill the friendly bacteria in your aquarium and are harmful to your fish. Therefore it needs conditioning.
  3. Once this water has been treated as per the conditioners instructions, (usually leaving it to work for 5 or more minutes for the chlorine and chloramine levels to settle) slowly pour it into your tank. Be sure to add each bucket one at a time and at a snail’s pace.
  4. Once you’ve filled your tank up to the desired level, your job is over.

Congratulate yourself on a job well done and admire your happy and healthy tank for a few minutes.

Common Water Change Mistakes

A sad looking cartoon goldfish in glasses

Take note, because these are some of the common mistakes that a lot of fish keepers make!

Firstly, don’t use fresh tap water to rinse and clean your filter, filter sponges, decorations etc.

Always ensure that you’re using your old tank water to clean these (as mentioned in our steps above), as this is where some good bacteria lives…and you don’t want to kill that.

Secondly, ensure you never add new water to your tank quickly.

New water has a different quality and make up to what your fish are swimming in, so add it in slowly to prevent shock, which can lower a fish’s immune system and increase their chances of becoming ill.

Thirdly, speaking of tap water, you really MUST treat it before adding it into your tank.

Tap water contains chemicals which could be harmful towards the good bacteria in your tank. These chemicals may also harm your fish, so it’s vital to not make this all-too-common mistake!

Finally, try not to change more than 40% of your tanks water at a time unless something drastic has polluted your aquarium and can potentially poison your fish. They are used to the conditions in the tank and changing too much in one go is stressful and can impair their immune system.

Also, it can break the nitrogen cycle by removing all the food (waste products) the good bacteria feed on, devastating the colonies that have built up over time. It’s best to leave some old water in there for stability.

Final thoughts on How to Change Aquarium Water

By now, we’re sure you understand the importance of correctly and regularly changing your aquarium’s water.

Remember, even though your aquarium might look clean, harmful chemicals go unseen by our eyes.

We recommend changing 40% of your tank’s water once per week, however this may change depending on the size of your tank and the concentration of fish.

By following our step-by-step guide to correctly making a water change, you’ll ensure that your fish are living in a healthy and beneficial ecosystem. This will keep them content, healthy and in their best condition!

Happy fish keeping!

Wendy Kathryn

Hi, I'm Wendy, the owner and creator of this website, an experienced fish keeper and avid student of the art since 2010. My aim is to help beginners avoid the many possible mistakes when getting started in this wonderful hobby.

4 thoughts on “How to Change Aquarium Water – Quickly and Safely for Your Fish”

  1. Hello my name Robyne I just broke down a 6.5 gl tank. Cleaned everything with hot hot water and then proceeded.to set it back up again got it all arranged and added the water I put it the easy balance let it sit and then checked the water it’s reading. That the nitrate level is extremely high and so is the nitrite level too so I took out some of the water and did It again it did NOT help at all what do I do ,? Here’s what I added to the water aqua safe, easy balance and I also tried Fluval waste control WHY isn’t any of these things working? It’s a fairly large tank for me to keep emptying it and it’s in the living room long. Haul with buckets of water PLEASE HELP

    • Hi Robyne,

      That is an odd one. First of all, I would question if your testing kit is actually accurate? it might be worth trying another one, just to be completely sure.

      What levels of Nitrate are we talking here? Have you tested your water straight from the tap, because nitrate is the most common groundwater contaminant and can work it’s way into our water supply, though it is monitored and acted on. However, I don’t know where you are, and what your water supply is.

      If your water supply is high in nitrates, you may have to resort to additives.

  2. My daughter got a beta fish for her birthday in a small bowl tank. The tank was set up with Betta water conditioner and distilled water. I have removed the beta fish now twice, I removed the beta fish and put back in the cup he came in. I think took 4 cups out of tank and put in a bowl. Next I cleaned the inside with water. I replaced the 4 cups I took out of the tank, I put 4 cups of the distilled water treated with the conditioner in and then the fish — 1. there is not as much water now in the tank — I would say it’s half full, do I add more distilled water to the fill it? 2. Today there seems to be a lot of little bubbles scattered at the top of the tank. I just want to figure out a system so the water height is the same and I know the beta will thrive. thanks.

    • Hi Sophia,

      It’s not good advice to use distilled water, as it’s simply too pure! During distillation, many trace elements and minerals are removed that are actually highly beneficial to fish. So please use tap water that has been treated with a ‘tap water conditioner for fish tanks’ instead. This will make it easy to manage. Simply remove some old water, and fill with the required amount of treated tap water, aiming for perhaps a 30% to 50% water change each time, with a frequency that depends on water parameters and when it needs changing.

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