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Aquarium water changes – How much? How often?

Aquarium water change

In this article we are going to talk through Aquarium water changes. How much needs to be changed, how often does it need to be changed and what happens if you don’t do water changes will all be covered.

Aquarium water changes

Aquarium water changes are vital to keeping a healthy aquarium and a very common question amongst new aquarists is, how often do I do a water change, and how much?

Unless you are lucky enough to have a tropical river running through your home, there is no escaping water changes if you want to keep a happy, healthy aquarium.

So in this article, I am going to cover the ins and outs of water changes, so that by the end of this you will understand why water changes are necessary and how to work out when you need to do them.


Why are water changes necessary?

Nitrate Build up

Aquarium water changes are necessary for various reasons. A primary reason is to remove waste.

Your filter converts ammonia and nitrite into less harmful nitrate, but nitrate often builds up which can still become toxic in high enough quantities.

The best and fastest way to reduce nitrate levels is with a water change. Changing water removes water that is filled with nitrates and replenishes it with new, fresh water, which then dilutes the nitrates that are left in the aquarium.

KH and PH maintenance

Another reason is to replenish the mineral content of your water. KH, or Carbonate hardness, is your waters system of buffering, allowing it to maintain a stable pH. This is vital as a changing pH value can cause pH shock in your fish, and lead to their death.

So that means that even in a heavily planted aquarium that does not see rising levels of nitrate, regular water changes are still necessary.


When to do a water change?

Unfortunately, no one can tell you exactly how often you need to do an aquarium water change except you and your tank.

This is because the only way to tell is by testing your aquarium, so instead, I am going to tell you how to work out how often you need to do water changes.

Once your aquarium is cycled, you will notice from using your trusty testing kit that nitrates accumulate in your water. In most cases, nitrate levels are going to be what dictates when you do a water change.

Nitrates are far less toxic than ammonia and nitrite but they are still a danger to your fish in high enough quantities.

Nitrate poisoning

The signs of nitrate poisoning may not be noticed until nitrate levels reach well above 100ppm+, but some fish are more susceptible than others. In most adult fish, nitrate doesn’t become harmful until it reaches very high concentrations but some are more sensitive.

Fry, juvenile fish and sensitive species can be affected by nitrate levels above 80ppm.

So, whilst it is not as toxic as other nitrogenous compounds, nitrate still needs to be removed from your aquarium and the best way to achieve that is with water changes.

Nitrate testing

Nitrate should be kept below 80ppm (API test), but ideally, it should be below 40. This is where that trusty test kit comes in:

You will need to monitor your aquarium after a water change and watch as the nitrate levels rise. This may mean testing every day, or every other day for a week or two. This will tell you how quickly nitrates rise in your tank.

Once you know how quickly your nitrate level rises, you should be able to calculate a regular schedule for removing them with aquarium water changes.

For more on Nitrates, Click here.


High Nitrates

If you find that your nitrates rise extremely quickly, then there are products that can help to ease the maintenance burden. Some filter media is designed to help to remove nitrates from your water by providing space for anaerobic bacteria. Live Plants help a great deal too!

But, if you would like to add some media that will help, I recommend Seachem Matrix, its affordable, lasts forever and it works, here’s an Amazon link if you think it would be useful to you:


Seachem Matrix – 1L Amazon.co.uk

Seachem Matrix – 1L Amazon.com


(As an Amazon associate, I earn on qualifying purchases).


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Planted tanks

As I mentioned earlier, heavily planted aquariums often do not see rising levels of nitrate. This is because plants absorb nitrate as food.

So how do you tell when you need to do a water change?

If nitrates are being absorbed by an abundance of live plants, then it doesn’t need to be reduced by water changes, but you will still need a water testing kit handy and you will still need to perform water changes.


Planted tank water changes

KH, or carbonate hardness is your waters method of buffering and keeping a stable pH. But over time, the carbonates in the water are used up and their abundance begins to drop.

Once the KH drops enough, the pH of your water can swing wildly as it is affected by everything else in the tank. As it is no longer stabilised by the KH your aquarium water will become far more acidic.

This is dangerous and can have serious effects on your fish and your filtration.

That means that if your tank does not produce rising levels of nitrate, you will need to keep an eye on your KH instead.

The recommended level is between 70 and 140ppm for a tropical, freshwater aquarium. This level should maintain a stable pH.

This may mean that you don’t need to do water changes very often at all. But it would be safest to keep them regular and change around 25% per week. If nothing else, it’s a bit of fresh, clean water for your fish.


How much should I change?

Many aquarists will tell you that there is a set amount of water that you need to change every time you perform a water change. This isn’t true at all.

Providing you have a mature filter and a regular maintenance schedule, there is no harm in doing 100% water changes, provided you keep the temperature the same. But really there is no need to do so.

The reason 100% water changes are not recommended is that it is difficult to keep the water the exact same temperature, and also, what do you do with the fish? It also isn’t necessary, unless things have gone horribly, horribly wrong.

80% weekly changes are fairly commonplace though. In tanks such as African Cichlid tanks, where aquariums are well stocked, nitrates can rise quickly so larger changes are required to keep them well in check.

You only need to view KaveMan Aquatics channel to see how well maintained his tanks are, he changes 80% per week with no harm to his fish or filters etc.


What happens with no water changes?

If you have a poorly maintained aquarium, that hasn’t had a water change for a while, then some different rules apply. Aquariums that have been left for a while may have developed what is known as ‘old tank syndrome’ along with some very high nitrate readings.

If this is the case, a large water change could shock your fish as the makeup of the water would change so drastically from what they have become used to.

Old tank syndrome is where an aquarium has been unmaintained and the KH value has dropped. This leaves the aquarium acidic as there is nothing to buffer the pH.

If fish have survived this, they may now be shocked by a quick pH change, so you need to return the aquarium to normal slowly with small 10% water changes daily, or every 2 days until it returns to where it should be.

A nitrate level of over 100ppm should be reduced slowly too, so as not to cause shock. A routine of 10% per day until the value is back to around 20ppm should work without issue.

I guess what I am saying is that there is no set amount of water to change, this again comes down to your nitrate levels and the use of your trusty aquarium testing kit. If your nitrate levels need to be cut in half after a week, change 50%. If they have only risen a small amount, change less.

A general rule of thumb is to change 25% per week. But this doesn’t work for everyone. Always ensure that you test your aquarium water yourself, so you know how quickly your parameters change.

If you add more fish or change your feeding routine, check it again to make sure your schedule doesn’t need to be adjusted.


Always ensure that you use a good quality dechlorinator when doing an aquarium water change, here is our top recommendation for water conditioners: The best aquarium water conditioner.


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Finding aquarium water changes tough?

If you’re finding that water changes are a lot of work, make it a little easier on yourself.

Have a quick read of our DIY Python hose ideas HERE. For as little as £25 you will never have to carry a bucket again!

About the author

James @Seriously Fishy

I am a fish keeping enthusiast with over 20 years experience. I currently keep American Cichlids (CA) which are my favourite fish to keep so far. I started Seriously Fishy as I noticed a large volume of people on various web pages looking for help with Aquarium basics. I created the first Seriously Fishy book to solve the issue in 28 pages, that led to the Seriously Fishy UK fish forum and blog.

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