Pond Filtration: What type of Filter System do you need.

There are two main types of filter that you would use in your garden pond to keep the water clear and the fish healthy, but you will need a filtration system for those filters to work.

Novice pond keepers and beginners think the filter system keeps the pond sparking clean and healthy by just removing physical dirt from the water but this is not the case.

Yes there are filters within the system that do remove waste from your pond but it’s not the full story, there are harmful toxins in the water that also need to be filtered out, read on to find out what the filters do, why you need them and how they are used in a filtration system.

Do you need to have a filter system in your pond?

No not all ponds need to have a filtration system, it all depends on the type of pond you have.

If you don’t intend to have fish in your pond there’s no reason to have a filter.

If you are building a wildlife pond with lots of plants you can choose not to use a filter, you could even get away with having a wildlife pond with a few fish, as long as there is plenty of plant life withing the pond to create a well-balanced ecosystem.

If you do decide to build a wildlife pond, it may take some time to get a good balanced nitrogen cycle working without a filter system in place, but it can be done, just give it time.

If you want to keep a good stock of fish in your pond you must realise they produce waste, both in liquid and solid form which goes straight into your pond water, so you will need to have some sort of filtration system to clean it up, as well as other debris that falls into your pond along the way.

What you need to understand, is a fishpond is a closed environment and all the water is trapped within the pond, not like natural ponds that have streams with fresh water passing through, add fish to the equation and that static pond water will soon become dirty and full of toxins.

The actual filter system that you will need depends mainly on the size of the pond, how may fish you intend to stock and where you plan to keep the system.

This can range from a small filter system for a small pond with goldfish right up to a large multi-chamber drum filter system for a large pond full of Koi, but the principles are the same.

What types of filters are there?

There are two main filters that are used in your filtration system each one having it’s own job to do to keep your pond well-balanced, clean and healthy for your fish.

Mechanical pond filters.

The mechanical filter is the first filter your water hits in a filtration system, these take out the large particles of dirt suspended in the water.

This first stage of filtration is very important to your system, it’s there to stop any large particles of waste getting to the next stage which is biological (I will get to that in a bit).

If large particles did get past this filter it could clog the biological filter, so it’s best to get this stage right or you will have problems down the line.

There are a few methods that can be used for mechanical filtration but the most commonly used is sponges of various grades which traps the large particles of waste.

For your larger ponds you may want to go for more mechanical filtration using a multi-chamber system that has different mechanical media in each container.

Mechanical filters need to be cleaned on a regular basis to ensure they don’t get blocked and reduce the flow of water in the system.

Biological pond filters.

Once the pond water has passed through the mechanical filters in enters stage two of the filtration system, the biological filter.

The biological filters job is to remove harmful toxins that are present in the water such as ammonia and nitrate.

Having a nice clear pond doesn’t mean the water quality is good, there could be toxins in your pond such as ammonia, nitrites and nitrates which cannot be seen, this is where the biological filters play their part.

These filters are designed to host large colonies of beneficial bacteria that convert the harmful and sometimes dangerous toxins into safe water for your fish.

Typical biological filter media can be anything from foam to plastic bio balls and bio glass to lava rock, but what they all should have is a large surface area for the good bacteria to grow on, the more bacteria the filter holds the more efficient the filter becomes.

For biological filters to work at their best they need a good mechanical filter to remove the solids before getting to them, a large colony of bacteria living on the bio media and a continuous supply of oxygen through a good flow of water to keep the bacteria alive.

It takes 4-6 weeks for a colony of bacteria to build up to a point where it reaches a good working level, so if it’s a new pond you don’t want to introduce fish until the levels are met.

Biological filters do not need to be cleaned unless there is a major problem, if you clean them you will wash away all that good bacteria that has taken so long to colonise and you will be back to square one.

Ultraviolet clarifiers.

Now I know these are not really filters in the way that mechanical and biological filters work, but they do play a part within the filtration system.

An ultraviolet clarifier (UVC) is designed to remove single cell algae, that’s the algae that cause your pond to go green.

The pond water is pumped into the clarifier and the ultraviolet lamp kills the single cell algae, once dead the algae clumps together and is collected and broken down by the biological filter or captured by the mechanical filter.

These UVC’s can be integrated within the pond filter or used as an inline standalone unit.

UV lamps have a short life and should be replaced at least once a year, if you are replacing your lamp the best time to do it would be the spring as your pond wakes up, that way you will have good UV protection through the summer months when algae is at its strongest.

Pond filtration systems.

Well now you know what each filter does it’s time to look at filtration systems.


A pre-filter is a simple mechanical filter that blocks large debris and solids before they are sucked into the pump.

Using a pre-filter is not necessary, but it can help to extend the life of the pump and improve its efficiency, just make sure you clean it on a regular basis to keep the water flowing.

Pond skimmer.

A pond skimmer is installed at the edge of your pond and will house your pond pump.

With the pump running the water is drawn to the skimmer bringing with it debris floating on the surface of the water.

The water with the debris falls through the mouth of the skimmer onto a coarse mechanical filter or a basket that captures all large debris that can be removed with ease, this system can be classed as a pre-filter because it catches the big debris before it reaches the pump.

If you are going to fit a skimmer it needs to be positioned at the opposite end to your waterfall or were the water is returned to the pond, this is to make sure you have full circulation of the pond water.

You can also use a floating surface skimmer that has its own built in pump, these are just placed into your pond and plugged in to your electricity supply, the skimmer will draw the floating debris into it’s basket, to clean just take out the basket, clean it out and replace.

These are great devices if you have a lot of falling leaves from nearby trees and plants or grass cuttings getting into your pond.

Another good benifit of having a skimmer is the pump is easy to remove for maintenance.

All in one pond pump/filter.

As a novice pond builder it’s easy to see why these look like the best and easiest filtration system to use.

After all they have a pump, filters and a UVC all combined in one small compact unit that you just pop in your pond and be done with.

Unfortunately all is not what it seems, yes they do what they say they will do, but they are only useful for a smaller pond or a pond that has very few fish.

If you have a small pond with a few fish then your in luck but anything bigger and it will struggle, that’s not to say you can’t use it alongside a larger filter set up.

Let’s say you have a full system with pump and filters but you fancy a waterfall using a separate system, you can buy a small waterfall pump that would just run the waterfall or you could use an all in one which gives you the added benefits of filters and a UVC.

Read our all-in-one pond pump review here

Flow through filters.

These flow through filters also known as box filters are perfect for the medium to large pond.

They are a simple design that contain the mechanical and biological filters inside the box, some even come with a UVC.

They are designed to sit outside the pond, but must be positioned above the water surface, the water is pumped through the box passing through the filters before flowing back into the pond through gravity.

The outlet pipe can be directed back into the pond or into a stream or waterfall, as long as the box is higher than the water feature it’s returning the water too it will work.

One of the problems of running this system is trying to hide the box, but it is a tried and tested method and is good value for money.

Waterfall filters.

These waterfall filters act in a very similar way to the flow through filters, water is pumped into the waterfall filter through mechanical and biological filters and back out through gravity.

The difference is the waterfall filter has a ledge that the water spills over to form a waterfall rather than having outlet pipes that you would direct into a purpose built waterfall.

They can be set at the edge of your pond so the water spills over the ledge straight into your pond giving a pleasing waterfall, still hard to hide though.

Pressure filters.

A pressure filter comes in a big round container that can be placed anywhere around or near to the pond.

Inside the pressure filter you would usually find mechanical and biological filters along with a UVC, but not all, you can get some with just mechanical filtration and some might not have the UVC.

They work by maintaining a constant water pressure (hence the name) and flow, being pressurised they are perfect for using with waterfalls or other water features as they can push water uphill to the top of any water feature.

They have an inlet and outlet for water flow plus another outlet for cleaning, the submersible pump in the pond pumps the water in through the inlet pipe which then passes through all the filtration and exits the outlet pipe.

The outlet pipe can be directed up to a waterfall or to another water feature or straight back into the pond.

Most have a cleaning system combined within the unit, this is when the cleaning outlet is used, a twist of a knob, a turn of a handle, or a pull/push on a handle depending on the make of pressure filter will clean the filters and the dirty water will be discharged through the cleaning outlet without getting your hands wet, they usually come with an indicator on top of the unit to tell you when it needs cleaning.

They don’t have to be above the waters surface like the flow through filters, so they can be placed anywhere, in fact you can bury them in the ground just leaving the top sticking out which is what most people do.

The great thing about burying a pressure filter is how easy they can be disguised so you wouldn’t even know they were there.

Check out our review on the best pressure filters here

Multi-chambered filter system.

Multi-chambered filters are just a series of flow through filters that are linked together through pipework.

The first chamber will have mechanical filtration, the pumped water passes through the foam taking out the large waste before moving on to the second chamber.

The second chamber is for biological filtration and you could pump into a third chamber also containing bio media.

They don’t come with a UVC but these can be added for full protection.

Shower filter.

These shower filters are not as common as the above filters, but they do a great job at filtering your pond water.

They are simply trays that sit on top of each other and each tray is filled with a filter media.

On top of the trays is a spray bar that takes the water from the pond pump and sprays it onto the top tray, it passes through the top tray into the next tray using gravity.

Once it has passed through all the trays it is dispatched straight into the pond giving a pleasing waterfall effect.

You can decide on how many trays you want and what media is used in each tray, for instance you could have a three tray system, the top tray filled with foam for mechanical filtration and the bottom two filled with pumice for biological filtration.

If you wanted too you can use all the trays for biological filtration as long as you used some sort of mechanical filtration first.

Cleaning would be done by turning of the pump and cleaning the mechanical foam filters in a bucket of pond water, replace the cleaned foam and turn the pump back on, there’s no need to clean the biological media.

Drum filters.

Another filtration system that’s not commonly used but if your looking for the ultimate mechanical filtration system this is one to go for.

They can be pump fed or gravity fed but if you go for gravity fed you need to have a bottom drain in your pond.

They are designed to remove the biggest particles right down to the smallest particles of waste.

Inside the unit is a drum that has a very fine mesh that catches all the waste, once it senses the filter needs cleaning the drum turns, it then uses water from the pond to spray onto the drum to remove the waste, the waste is discharged through a separate outlet. This is all done automatically.

Once the pond water has passed through the drum filter it needs to go to a biological filter in this case a moving bed module.

The moving bed module is filled with Hel-X media which is a very efficient home to friendly bacteria.

The reason the Hel-X is moved around is to give it a a permanent supply of oxygen for the bacteria and it also acts as a self-cleaning function.

As the media moves about it knocks of any dead or old bacteria making way for new bacteria to grow.

There is one last module that can be added to the system which can house extra bio-media or if using the system as a gravity fed system houses the pump and UVC.

A UVC can be added to the inlet just before the drum to kill the one cell algae and the drum will remove the clumped up algae.

So which is the best filter system for you.

First things first, there is no such thing as over filtering a pond, go for the largest filter system that you can, you can never have too much filtration.

On the other hand your pond could get into real trouble by under filtration, even if the filter system says it will filter the size of your pond always go larger, going larger will future proof your pond as well as giving greater results.

Before buying a filter you really need to know what it’s needed for, it makes no sense to go for the ultimate filter system if you are just going to have a couple of goldfish in your pond, and the opposite of have a large selection of Koi with the minimum amount of filtration.

If you have a small pond with just a couple of goldfish then you could probably get away with the all-in-one pond/filter but keep an eye on the water and test regularly.

For a medium pond the flow through or pressure filters are going to be a good solution, just make sure they are more than adequate for the size of pond and the amount of fish you want to keep.

For a large pond with lots of fish you can get away with the very large flow through filters or even better the muti-chamber filters as well as the pressure filters, if you find one filter starts to struggle you could add extra filters to compensate without having to start again. You could have two pressure systems or a pressure system to feed the pond and a flow through system to feed a waterfall.

For the dedicated Koi pond with lots of large fish you will struggle unless you have a good filtration system, the drum system would be the way to go but it is expensive compared to other systems.

Whatever system you go for you need to test the water regularly to make sure its doing it’s job, just because the water is clear doesn’t mean the water isn’t full of harmful toxins.

I hope I have answered all your questions and good luck with your pond.