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Filtration and Filter Placement


Aquarium filtration and filter media


Filtration has to be the most important part of any aquarium set up except for a Walstead setup.  Without filtration, all you’d be left with is a box of dead fish in some dirty water.  So it’s certainly of massive importance.

First off your filter needs to be powerful enough for the volume of water you have in your tank. As a general rule of thumb, you need the entire contents of your aquarium to be cycled through your filter at least 5 times every hour, unless you have a very slow swimming fish like a beta which needs less flow.

So, as an example, if your aquarium is 100 litres, you need a filter that is rated for 500 litres per hour (LPH) or more.

Some filters will claim that they can clean more water than this, this may be the case if you set up your media correctly, but you may find you need some additional flow to prevent build ups in the tank.

Most filters come with media fitted, but it is important to understand what each product does.  There need to be 3 types of filtration going on in your filter in order to keep your aquarium clean and your fish healthy:

  1.  Mechanical filtration is used to remove debris from the water column, this may be bits of fish food, fish poop or plant matter.  Most commonly this is made up of sponges of varying density (finer sponge collects smaller particles).  This helps to keep your water clear from larger pieces of debris.
  2. Biological filtration is what we talk about when you are cycling your aquarium, this is where waste (ammonia) is broken down to prevent it from poisoning your aquarium inhabitants, ammonia becomes nitrite becomes nitrate.
  3. Chemical filtration removes dissolved matter from the water column.  This isn’t always necessary for a healthy aquarium but it will help to reduce any odours and keep the water a little clearer.  The main use of chemical filtration is to remove medication from the water after any treatment.

When it comes to media, most experienced aquarists have preferences on what they use as there is a range of media out there for each type.

For mechanical filtration, a sponge is a sponge, as long as it doesn’t fall apart and it fits your filter correctly it should do its job.

Chemical media is similar, it is all very much the same thing in a different package.

There are however, a million biological media available though and some are far better than others. Some of my favourites include;

Seachem Matrix

Fluval Biomax

Biohome ultimate

Seachem Purigen (This sometimes appears as Chemical filtration but it absorbs nitrogenous waste, so to me, it’s bio)


When it comes to Bio media, the surface area is really important, more surface area means more space for bacteria to live, but some media, like Biohome is also extremely porous which means bacteria can actually live inside it too.


Filter set up

Not long ago I realised I had set my external filter up incorrectly for my aquarium.  When I purchased an external filter for the first time I read the instructions and set it up as they stated.  Ever since then I’ve been setting them up the same way.

It was only a few months ago that I noticed I had dead spots (areas where debris would build-up) in the 5 x 2 x 2 aquarium in my living room, despite it being filtered by a Fluval FX5 (they go forever!) plus powerheads to increase flow.


So where have I been going wrong?

Once I noticed the issue in my aquarium I initially started moving the powerheads around.  After trying it out in a few different places the problem persisted or just moved.

Then I had a thought, I use powerheads to push water across the surface of the tank, it then returns along the bottom.  This was working fine, so then I thought about my filter.

My filter is also set up to push water across the surface of the tank, but it was set up in the same way that I set up my first ever external, with the intake and outlet at the opposite ends of the aquarium.

This is why I had a dead spot at the end of the aquarium where the filter outlet and powerheads were positioned.


Seems like common sense?

Now that I’ve written this out, it certainly seems like I had a common-sense failure.  If I had really thought about it, I probably would have worked this out years ago!  This was just the way I had always done it and I had previously not noticed any issues.

I must admit though, my ‘aquarium OCD’ has increased over the years so it may just be that I wasn’t as concerned with a small build-up of debris.

It seems though, it’s pretty common and many people set their filters up this way, and like the video above shows it’s often done by the companies who make the filters, so I feel a little better.

Anyway, as you have probably guessed, the solution here was to move my inlet and outlet to the same end of the aquarium, this means that all the water is pushed across the top of the tank, hits the other end and returns along the bottom, taking any debris (poop and bits of plant mostly) towards the filter intake.

The important thing to take away is to check the flow in your aquarium, is it fully circulating around your tank? Are there dead spots?

For anyone new to external filters, here’s a video to show you how to install one, it is specific to APS filters but the concepts are all the same:


Another key factor in keeping a healthy aquarium is water chemistry. You can learn more about this here.

Hit the comments and share your setups and preferences!

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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