Our Guide To Garden Pond Filters

All ponds need a filtration system to keep the water clean, clear and healthy. Stocking levels of fish are much higher in a garden pond than in the wild.

We have put together this FAQ feature on garden pond filters to address some of the questions asked by people new to pond and fish keeping. 

Do I Need A Pond Filter?

Whether you need a pond filter depends on the type of pond you have. If you want fish in your pond then I would say 'yes' every time. Pond fish create lots of waste and the only totally effective way to get rid of this waste is with a garden pond filter. If left unfiltered, the water can become polluted, poison your fish, cause the water to go cloudy and smell. This is not what you want from your garden pond.

What Do The Numbers Mean On Pond Filters?

The numbers on filters normally indicate the maximum pond size they are able to filter, however you must be careful with this because these numbers usually mean WITHOUT fish. Some filters, however, like the Hozelock Bioforce Revolution show the pond size they can handle WITH fish. This is a new idea by Hozelock to make things easier for the consumer to understand rather than giving what some would consider misleading information in an attempt to make the filter look more efficient.

You will also find that pond filters will list a maximum flow rate, this is the maximum rate of water per hour the filter can handle. You should pay careful attention to this rate when choosing your pump. Too large a pump can overflow a box filter or blow the seals on a pressure filter, too small a pump will mean the filter is unable to efficiently filter the pond.

Which is Best, an Underwater, Pressure or Box Pond Filter?

The type of filter you choose should be based on your pond and not the price, as ultimately you could end up buying a whole new filter system after initially buying the wrong type. I've experienced this scenario with many customers who have commented to me that they wish they hadn't made this mistake.

Hozelock EasyClear Underwater Pond Filter1. Underwater Pond Filters

These are good for small to medium sized ponds with low fish stocking levels, they’re quick to install and will usually come with a built-in UV clarifier.

The issue with these filters is that for cleaning you need to get them out of the middle of your pond, not something you will want to do when it’s freezing cold outside. They also don’t do well in big ponds or ponds with many fish as they have a very small filter surface area.

2. Pond Pressure Filters

Hozelock Bioforce Revolution Pond Pressure Filter

These filters are great if you want to hide the filter but require something a little more substantial than an underwater filter. As they are pressurised they can be located below the water level, for example below and behind a waterfall. 

The issue with pressure filters is that they do need more maintenance than a box filter but not as much as an underwater filter. The more expensive pressure filters tend to have easy-clean options which counter this problem though, which has been a great innovation for the aquatics industry.

3. Pond Flow-Through Box Filters

Oase Biosmart Flow-Through Box Filter for PondsThese filters are probably seen as old fashioned but you can’t beat them when it comes to the sheer volume of water they’re able to filter and they’re not normally as expensive as the equivalent pressure filter.

The large surface area allows for lots of foams and filter media and a nice big UV clarifier if yours has one, and this will keep your pond extremely clean.

The downside is that they’re bigger and obviously harder to hide, especially if you don’t have a huge garden. They are easy to clean though and many of the more expensive box filters come with easy cleaning options. 

A box filter will overflow if the pond pump is too powerful for it so I'd say be extremely careful when checking the maximum flow rate of the box.

Do I Need a UV Clarifier on My Pond Filter?

A UV clarifier will eradicate suspended algae, which is informally known as green water. We'd always recommend a UV clarifier because it prevents this unsightly algae. Whether or not to have one really depends on whether your pond is well shaded or not, as green water is caused by constant sunlight so if it's unshaded I'd definitely say 'yes'.

You will need to change the UV lamp every year, this is because the UV output will diminish over a period of 6-9 months of constant use.

How Do I Clean My Pond Filter?

Although there are different ways to clean individual filters, the premise is always the same, never wash your filter out using tap water. Try to use water taken out of the pond if possible, this is because you are not trying to clean the foams and media but rather just unclog them. Using tap water will kill the beneficial bacteria that breaks down the waste.

The easy clean systems built into pond filters will normally use the water from the pond to clean the foams and then the dirty water, which contains useful nutrients, is sent into a flower bed. 

Is Bigger Always Better When it Comes to Pond Filters?

Yes, you can never over-filter your pond water so the larger the filter the better. If you pick a larger filter it will mean less hard work for the filter and less frequent cleaning, meaning less work for yourself.

Types Of Filters

Mechanical and biological filters, together with UV clarifiers, will cleanse the water to create the best possible environment to encourage a thriving population of fish and plant life.

Mechanical Filters

Dirty water flows through the filter, foam or other media that captures debris. Filtered clean water then returns to the pond.

Biological Filters

This process involves a filter medium that contains beneficial bacteria that cleanse the water.

UV clarifiers

These are included in many filtration kits and available separately with the UV helping to eradicate 'green water' by eliminating algae.

For ponds up to 10,000 litres, box filters are the best choice, they may contain a clarifier and can be biological or alternatively mechanical. If you prefer, some boxes can be hidden underground or you might choose a filter that can be immersed. 

Pressurised units allow you the option of powering a waterfall or fountain. More expensive filters feature advanced technology, can handle larger volumes of water or offer functions that are labour saving and can significantly reduce running costs. These include self-cleaning systems and power saving devices that enable the UV filter to switch off when not required.

Filter Capacity

When choosing a filter, consider what is most appropriate for your particular requirements. Follow the manufacturer’s recommendations on the filter’s capacity and the volume of water it can process. Remember that water should be circulated at least once every two hours i.e. a 2000 litre pond will need a filter that generates a flow of 1000 litres per hour.

Don’t forget that if you have fish, take into account how many and how large they are, as this can dramatically affect the specification that is required, as can factors such as whether you want the clean water to be pumped to fountains or other ornaments.

Also think about practicalities such as the ease of maintenance, perhaps a self-cleaning system is preferable. Running costs must also be considered, both in terms of the amount of power required and the durability and longevity of the product parts.

There is no doubt the range of pond filters on the market is daunting and can be confusing. Basic facts such as the volume of water, number of fish etc are vital components in making a decision, the rest is probably personal choice, preferably reinforced by the advice of a specialist retailer.