Maintaining a healthy pond with a flourishing variety of pond fish requires the use of a pond filter. Pond filters are a vital piece of pond equipment that are available in a variety of models that keep your fish healthy, as well as your pond water. At Swell UK, we have a wide range of pond filters available that are suitable for all kinds of ponds, including anything from a small wildlife pond to a large Koi pond. Here is everything that you need to know about this crucial piece of pond equipment…

How does a pond filter work?

Similarly to a filter in an aquarium, pond filters remove waste products from your water column. Ponds can quickly become overrun with waste from decomposing plant material or fish waste. If untreated, this waste accumulates quickly and begins to rot, causing your water column to fill with excess ammonia and nitrite that can degrade your water quality and poison your fish.

Pond filters use mechanical filtration to remove waste from your pond water, and biological filtration to break the waste down. The internal filter foam has a large surface area that can allow beneficial bacteria to colonise. Beneficial bacteria feeds on any excess ammonia or nitrite in your pond and turn it into nitrate, which is absorbed by growing pond plants, removing it from your waters.

Some garden ponds do not require the use of a pond filter. Wildlife ponds don’t house fish, meaning there is no need for a filter to break down waste. On the other hand, a pond filter is an absolute necessity in a Koi pond, to create a healthy environment that will aid in the support of a growing Koi population.

There are many different varieties of pond filters to choose from that differ in size, shape, form and function. Each filter serves a specific purpose, with unique strengths and weaknesses, but they all help to improve water quality and clarity. To find the best pond filter to suit your needs depends on the strength of your pond pump, and the size of your pond.

Pressure Filters

Pressure Filters (or pressurised pond filters) are filters that keep the water leaving the filter unit pressurised.

Pros:

• Can be used alongside water features such as: waterfalls, streams, and pond fountains
• Can be placed below the water level of your pond, or even partially buried
• Large filtration capacities for medium or large ponds
• Some filters, such Pressure Filter Premium come with built-in UV lighting
• UV lighting prevents green water, and eliminates dangerous pathogens, bacteria and viruses

Cons:

• Can become clogged with waste solids, requiring more maintenance than other filters, although many filters come with internal cleaning mechanisms

Box Filters

Box Filters (or flow-through pond filters) are one of the most popular filters for ponds. These filters work alongside your pond pump to pump water under pressure, into the box filter. Water then flows through the box filter, draining out with gravity.

Pros:

• Has huge filtration capacities
• Often come packed with large amounts of filter foam
• Extra space for your own filter media like alfagrog and carbon
• Easy to maintain and don’t need as frequent cleaning as other filters, like pressure filters, but will need mucking out each season

Cons:

• Limited placement options, outlet must sit above water level
• Can’t be used in conjunction with pond fountains or other water features that require a constant supply of water travelling under high pressure

In-Pond Filters

In-Pond Filters are very simple and straightforward pond filters, that typically combine a filter, pond pump and UVC lamp in one unit.

Pros:

• Easy to install
• Fantastic choice for first-time pond keepers
• Perfect for miniature ponds where a large filter would be unnecessary
• Excellent all-in-one solution for small ponds

Cons:

• Small units that can’t support a large number of fish
• Doesn’t produce the water flow necessary to keep water properly circulating in a medium or large pond

Koi Pond Filters

Koi Pond Filters are very large pond filtration units that are specifically designed to house messy pond fish such as Koi and goldfish. These filters work by pumping water into a huge drum that keeps specialised media (K+ media) in constant suspension.

Pros:

• Keeps your pond clean and water clear
• Prevents excess nutrients that may promote algae growth and green water
• Increases the frequency of water-media interactions
• Boosting the filter’s biological and mechanical filtration capacity.

Cons:

• Limited placement options, often housed in sheds to prevent delicate components becoming damaged by the atmospheric elements
• Takes a lot of time to clean

Is there a difference between pond filter and pond pumps?

I may be a challenge as a beginner to tell the difference between a pond filter and a pond pump. Pond pumps supply pond filters with water, whereas pond filters filter excess nutrients and pollutions from the pond, allowing you to have healthy water. Pond pumps and pond filters are used together to form an excellent filtration system that support a healthy pond full of life, with regular maintenance.

Some pond filters are referred to as all-in-one filters, typically with a built-in pump that allows your filter system to become a plug-and-play unit. Another pond filter that falls between a pond filter and a pond pump is a pond skimmer, which float on the surface of your pond. Pond skimmers strain floating debris, such as leaves and uneaten food into the filter basket using the suction force that is created by a built-in pump.

What pond filter do I need for my pond?

There are many things to consider when choosing a pond filter for your pond. The factors to consider are the volume of your pond, the amount of fish you have, the species of fish in your pond, the space you have available, and any water features you have in your pond, including fountains, streams or waterfalls.

What size pond filter do I need?

As you cannot ‘over-filter’ a pond, it is always recommended to purchase the biggest pond filter possible. If you have the space and budget for a larger, more powerful filter you should always go for that one. In-pond filters are suitable for medium-sized ponds, pressure filers, box filters and Koi pond filters are quite large. The largest models in the range of pond filters are powerful enough to effectively filter large ornamental ponds that are full of fish.

What pond pump do I use for my pond filter?

Not every pond keeper has the budget for a large powerful filter, or you may not require one if you have a wildlife pond or small pond with very few fish. If you’re looking for a filter that’s perfect for the size of your pond, you should select a filter that has a capacity that matches the volume of your pond. You should also use a pond pump that is able to turn over the total volume of your pond at least once, ideally twice, per hour.

How do I choose a pond filter that is suitable for my water feature?

It is important to consider your water features when choosing a pond filter. If you don’t have any water features, your choice of filter isn’t really limited. With water features, a pressure filter may be the best option. As pressure filters keep water under pressure, it can be easily pumped up to water features like waterfalls, streams, and pond fountains, meaning you can place your pressure filter below the water level of your pond, or even partially buried.

For more advice on how to choose the right pond filter for your pond, check out our handy help guide on the topic here.

How to calculate the water volume in my pond?

Calculating the volume of your garden pond is easier than it sounds, all you need are some measurements and a simple equation. Measure the average length, average width and average depth of your pond. These measurements don’t have to be absolutely accurate, rough measurements will work fine. Add the numbers to the following equation:

• Average length (m) x average width (m) x average depth (m) = your pond’s volume in cubic metres. Then, multiply the cubic metres by 1,000 to get your pond’s volume in litres.
• Example: (5 x 5 x 1) x 1,000 = 25,000L

There’s a lot of information here to get you started, but for more advice and tips be sure to read our useful pond help guides. Also, check out our range of other garden pond essentials, such as pond pumps, pond liners, pond fish food, pond treatments, and pond plants.