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Aquarium water purifier – Gimmick or miracle?

aquarium water purifier carbon cube

There are a lot of these so-called ‘Aquarium water purifier cubes’ on the market these days, but what are they and what do they do? Do they do what they say they can?

Aquarium water purifier – Carbon cubes

It seems to be a common occurrence recently to come across posts on social media advertising ‘Aquarium water purifier cubes’.

I have seen a lot of comments on these posts with many people asking things along the lines of, ‘What are they?’ and ‘Do they work?’

Both are great questions!

So here we’re going to take a look at what they actually are and what they do…

Based on what the ads say, these aquarium water purifier cubes can do some amazing things.

They say they can be placed into the aquarium and that you won’t need to clean the tank or change any water, yet your tank will remain clean and clear.

They are advertised as being renewable with a simple rinse under the tap and say that they will keep your water crystal clear indefinitely.

Sounds amazing right?

It certainly does, but what is this ‘wizardry’ and does it work?


aquarium water purifier water purifying cube carbon cube 3

Aquarium water purifier – what is it?

These products are advertised as ‘aquarium water purifier’ cubes and are stated to remove a host of compounds from your aquarium water.

The long list of stated abilities would probably make these ‘aquarium water purifier’ cubes one of the worlds greatest achievements in aquarium water filtration – if they work.

After looking at a few of these products, I can tell you that they are made up of layers of plastic and activated carbon.

These products are advertised as being capable of removing a long list of compounds from your water including;

Ammonia, Nitrite, Nitrate, Heavy metals, Nitrate bacteria (whatever that is?), various other bacterias and phosphate.

They also claim to maintain the pH levels of the aquarium water and say that the cube can be refreshed with a simple rinse.

That’s a long list of effects for a little purifier cube and maintenance does sound really easy!


aquarium water purifier water purifying cube carbon cube 2

Carbon cubes – Aquarium water purifier?

After having a good look around at these products, it seems that what they are is a simple cube that contains activated carbon.

Activated carbon is already in use in the aquarium trade and has been used in water filtration for a long, long time.

Activated carbon can be bought by the box to add to your filter if required.

It tends to be pretty cheap but needs to be replaced every 2 – 3 months, depending on stock levels/feeding etc.

But can it have all these effects?


If activated carbon did do this, then there would be no reason to add this weird looking cube to our tanks. We could just add a bag of it to our filter.

Activated carbon certainly has its uses in the aquarium and it is frequently used inside our filters.

Really though, it isn’t necessary in a healthy aquarium and its main use is for the removal of some medications after treatment.

Activated carbon can adsorb organic compounds that cause odour and tastes in the water, it can also adsorb chloramine, chlorine, phosphate and some medications.

It cannot adsorb ammonia, nitrite, nitrate, heavy metals or any bacteria.

Activated carbon does not maintain pH levels either. In fact, it is more likely to increase pH as activated carbon is naturally alkaline.

Some activated carbon is washed in acid to prevent this, but it still does nothing to maintain the pH of the water around it.

Not looking good for these ‘magic aquarium water purifier cubes’.


aquarium water purifier carbon cube

Rinse to refresh?

Activated carbon has a huge surface area, but as compounds are adsorbed from the water that available space is filled up.

This means that after a while, the activated carbon can adsorb no more and needs to be replaced or renewed.

These carbon cubes advertise a simple refresh process that just involves rinsing the cube under the tap. One site even recommended washing it in soapy water (please do not do that!).

If activated carbon can be renewed by a simple rinse, why have aquarists everywhere been replacing it every three months?

….Because it can’t. 

The renewal of activated carbon is quite a bit more complex than a rinse under the tap and involves high-temperature thermal processes to remove the compounds that have been adsorbed. Not something that is easily achievable at home.

I suppose you could try and cook it in the oven? That wouldn’t go too well with the plastic in these cubes though.

After the renewal process, up to 95% of the activated carbon will have been refreshed, so eventually it still needs replacing anyway.

That means that rinsing this cube will not renew it. You will need to buy a new one every 2 – 3 months to retain the limited effects that it has.

So, it seems that the aquarium water purifier can’t quite do what it says it does.

For a full refresh of what these companies are saying this thing can do, see the video below. However, keep an eye on the tanks. Supposedly this cube in the video is in a small aquarium for 24 hours, yet the fish appear to change several times, who knows what is going on there?

What a load of $%#&!

Some Video evidence – Aquarium water purifier

So, is what I’ve been saying correct?

I just came across this video from a guy known as the ‘Guppy Guru‘ on Youtube. He tested one of these cubes in what is a pretty small aquarium over about 44 hours, here are the results:


The Verdict – Aquarium water purifier – Carbon cubes

As great as these little cubes sound in the ads, I’m sad to say that they are nothing but a gimmick.

These plastic cubes are nothing new to the aquarium, they simply contain activated carbon.

Activated carbon is a product that has been available and in use for a long, long time in both water filtration and indeed the aquarium hobby.

If activated carbon was capable of the effects that these cubes claim it is then we would know it by now and we would all have a bag of it in our filters.

The truth is, these cubes are not at all capable of what they claim to be.

Whilst activated carbon can have its uses in the aquarium, it is not capable of removing nitrate, nitrite, ammonia, heavy metals or bacteria from the water column.

In fact, it is capable of only a few of the claims that have been made by the companies selling these ‘aquarium water purifiers’.

It is certainly not a substitute for good aquarium maintenance.

So, for those looking for a product that means water changes are not necessary, the search continues.

In the meantime, take a look at our guide to making a cheap DIY water changing system that means you don’t need to carry a single bucket!

If you were looking at these aquarium water purifiers because you have cloudy water, take a look at our guide to filter setup.

These cubes will only mask any issues with cloudy water etc at best. So a far better solution is to fix the issue, rather than throw a cube of carbon in and hope for the best.

There isn’t any substitution for good aquarium maintenance, but there are certainly ways to work smarter.

Unfortunately, this magic wizardry in the form of a cube is just a well-marketed (in some cases), more expensive version of activated carbon, in short, it’s a scam……..

The marketing used to sell these is pretty dangerous in my opinion. Any new aquarist who believes that this so-called aquarium water purifier will replace the need for water changes will run into problems very quickly.

aquarium water purifier carbon cube



Frequently asked questions:







Spring Well water filtration systems

Water tech online – the basics of activated carbon adsorption

Water tech online – Extending the life of activated carbon

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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