Tackling the external filter - a beginners guide

Hi Guys,

Most of us start our with small manageable aquariums with internal filters and carrying out maintenance on these aquariums was fairly straight forward. 25% water change and a quick rinse of the sponges inside your filter and your done. While the simple life is always good, its just typical of us to want something bigger and better so these easy manageable setups generally get swapped for a bigger aquarium. With bigger aquarium comes bigger fish and bigger fish require more filtration. Suddenly your simple little internal filter just wont cut it anymore and your looking at upgrading to an external filter. For a lot of beginners this can be quite a daunting experience.

I remember my first Fluval external filter. I remember that noise when I switched it on without filling it with water. I also remember flooding the cabinet several times in the process of filling or "priming" it too.

Over the years, Ive had different filters. Wet/Dry ones, Hang on Back ones and internal ones. The best by far is the external filter. They save on space inside the tank and also distribute the water over a wider area so water movement is always good.

Ive learned that most external filters work on the same basis that they draw water from the tank via the intake pipe, through the different filtration media and back up the outlet pipe and into the aquarium.

Over the years, Ive become used to setting up and cleaning the external filters so I thought I would share with you some easy and straight forward steps to make sure your first attempt is successful.

First of all you have to buy the correct size of filter for your aquarium. The general rule of thumb is that a freshwater aquarium should  have a turn over rate of five times the total water volume every hour. In other words, If you have a 200l aquarium, you should have a filter with a minimum capacity of 1000lph (litres per hour).

The next important thing to consider is where your placing your external filter. In order for this type of filter to work, it needs to be placed underneath or to the side of your aquarium and lower than the water level. This is because these systems employ gravity to assist with the syphoning of the water from your tank. Inside the cabinet or on the floor to the side of your aquarium is normally okay.

When you get your new filter and unpack it, you might be a little overwhelmed. Compared to a small internal, the canister filter has lots more bits to put together. But don't worry, they are just bigger versions of the same thing. Your new filter should come with new media (the stuff inside that helps clean the water), Tubes to move the water to and from your aquarium, the powerhead/pump to run everything and some filters come with spray bars to distribute the water, some have other connections to give different effects.

The first step to assembling your filter is placing the media in the trays in the correct order. Each manufacturer designs their filter differently so read the instructions to work out if your filter pushes the water from the top of the canister to the bottom of the canister and some do it the other way round. The method is the basically the same either way. Where your water enters the canister, it will travel through the media and then back into your aquarium. I would recommend placing the sponges in order of the Course, medium, fine. If you have the fine floss (white stuff) it should be placed after the finest sponge. Your aim is to catch anything that is sucked into the canister  before it gets to the biological media which comes in different forms. Ceramic, plastic and sintered glass. The media allows the beneficial aerobic bacteria to form, which consumes ammonia and nitrite, producing nitrate as an end product which is used up by plants or exchanged with the atmosphere.

Once you have the media in their trays, placed into the canister its time for the working part to be placed on top. You usually find that this will only fit one way but just be careful not to place it round the wrong way and force it!

On my corner tank, both the intake and outlet are beside each
other. On rectangular aquariums, they can be at the opposite
sides of the aquarium
The next step is to assemble the pipework, spray bar and intake pipe. I like to place the intake grill about three inches form the substrate. If your installing a spray bar, have the holes pointing slightly up so it keeps the surface of the water moving continuously. When your happy with the positions of both, attach the tubing the each of them. Its common for the tubing to be difference sizes with the intake piping being thicker than the outlet. This is very helpful when attaching the tubing to the canister connections. When measuring the tubing, your looking to have as little as possible, with enough excess to allow you to easily pull out the canister for maintenance. Try and avoid a big dip in the tubing as this can cause a collection of debris over time and lead to a loss of flow. 

Attaching the tubing can be quit difficult depending on the make and model of your canister. In my experience, Fluval, Betta and Hydor are all fairy straight forward but Eheim involves heating and shrinking which can prove tricky.

With everything connected and ready, the final step is filling it with water. NEVER fill it with water from the tap. Most canister filters are designed with a manual priming function. This is basically a big button or pump on the top that you push rapidly. This creates suction witch draws water from the aquarium into the canister. During this process, the air from the canister is pushed back up the tube and into there aquarium so when your priming, keep an eye on the aquarium and look for the bubbles. When the bubbles stop, your ready to plug the power in and begin. 

Almost every time you do this, you will get a little bit of noise. This should stop after a few seconds. if not, unplug the power and try priming some more. 

When it comes to cleaning your external filter, it depends on the fish you keep. Large high waste producing fish like large cichlids or catfish are demanding on any filtration system so its recommended that you clean out the sponges every couple of months unless your keeping some real monsters.

If your considering making the upgrade to the external filter then go for it. Our Facebook group is a great place to get support if you need it.

Here are some fantastic tips put forward by some of our members.


  • Put the bio balls in the bin and get as good a filter media as you can! - Laurin McLaren
  • Buy a decent one and do a bit research get more lph than you need as it will slow down in use - Colin Logan
  • Stack your media correctly - Carl Bethell 
I hope this clears up a few questions or fears that you might have had about the external filter. If you have any other advice or questions surrounding external filters, please comment below. Also please subscribe to our blog to keep up to date.

Bye for now


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