Garden Pond Filters FAQ

David Coton has put together this FAQ feature on garden pond filters to address some of the questions asked by people new to pond and fish keeping, and hopefully, make it easier for others to make their choices.

Created by David Coton on Tuesday, 4th of March, 2014.
Updated on Monday, 30th of April, 2018.


A stunning garden pond during spring.

Do I Need a Pond Filter?

Whether you need a pond filter depends on your pond and what you’re planning to use it for. If you want fish in your pond then I would say 'yes' every time. Pond fish create lots of waste and the only real 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 too. This is not what you want from your garden pond.

What do The Numbers Mean on Pond Filters?

The numbers on filters will normally mean 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 and it will 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 had been told the facts in the first place so they didn't make the 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 if 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 too.

Our Pond Filter Range

You can view our full range of pond filters here.

Garden Pond Pumps FAQs.

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