The Difference Between Box Filters & Pressure Filters

May 10, 2014

Hints and Tips Pond Filters Pressure Filters

A high-quality pond filter is essential for keeping the water clean and healthy, especially if you keep fish. With many types of pond filtration available, this guide focuses on two common types for small to medium-sized ponds: box filters and pressure filters.

Understanding the fundamental differences between a pond box filter and a pond pressure filter will help you choose the right system for your pond's needs.

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Key Takeaways

Box Filters: Best for small-to-medium ponds where the larger filter box can be placed above the water level. They offer longer contact time with the biological filter media and use gravity for water return.
Pressure Filters: More compact and offer flexible placement, as they can be partially buried or used to feed elevated water features. They often include convenient, built-in backwash systems for easy cleaning.
Filtration: Almost all modern pond filters combine mechanical filtration (removing solid debris), biological filtration (using beneficial bacteria to manage the nitrogen cycle), and an integral UVC (ultraviolet clarifier) to control green water algae.
Choice: Your decision should be based on your pond's size, its fish load, your garden's landscape constraints, and your maintenance preferences.

Do I Need a Pond Filter?

If you plan to keep fish, a filtration system is essential. Fish produce waste, and the best way to process this waste is with a pond filter. An unfiltered pond will quickly become polluted, leading to cloudy water, potential odours, and an unhealthy environment that is detrimental to your fish. Do not leave a pond with fish unfiltered.

What Type of Ponds Are Box Filters and Pressure Filters Best For?

Both systems are suitable for small to medium-sized ponds.

Box Filters

We generally advise choosing a box filter for small to medium-sized ponds. This classic system provides longer contact time with the biological filter media (the surfaces that host beneficial bacteria), as the water is pumped through a physically larger unit compared to a pressurised filter. This longer contact time promotes a more stable nitrogen cycle (the process of converting fish waste into harmless nitrates).

Pressure Filters:

We recommend considering a pond pressure filter if your installation requirements do not allow for a bulkier box filter, or if you need to run water to a high point.

Box Filter vs Pressure Filter: Installation and Placement

The main difference lies in how water leaves the unit.

Box Filters

Box filters are not sealed. They return the water to the pond by gravity, which means the filter must be installed above the pond’s maximum water level. Many people choose to hide their box filter by installing it at the start of a waterfall or rockery, allowing the returning water to create a natural feature.

Installation

You will need a pump, pond hose (often called polyhose), and hose clips. The hose must run from the pond pump up to the filter inlet.

Pressure Filters

These are sealed units, meaning the water remains under pressure as it leaves the filter. This pressurised return allows them to feed a higher positioned watercourse, water feature, or even another point in the pond, giving you much more flexibility in positioning the unit.

Installation

A commonly used option for a neat finish, pressure filters are extremely easy to install and can be partially buried in the ground adjacent to the pond. Most pressure units have three hose connections: the inlet (from the pump), the outlet (to the pond/feature), and a backwash outlet (for cleaning).

Are Pond Filters Easy to Clean?

Regular cleaning is vital for the health of your pond.

Pressure Filters typically need to be cleaned more frequently than box filters, especially in ponds with a high fish load.

Many models feature a built-in backwash or easy-clean system that lets you quickly clean the internal sponges in a matter of minutes by turning a valve or lever, without opening the unit.

If your pressure filter lacks a backwash function, you will need to open the unit and clean the media manually.

Box Filters should be cleaned approximately every 2–4 weeks to ensure the filter media performs to its full potential. The method usually involves removing the filter sponges and rinsing them in a bucket of pond water.

How Do Box Filters and Pressure Filters Work?

Almost all pressure filters and modern box filter systems combine three stages of cleaning to provide both clear and healthy water:

Mechanical Filtration: Removes large debris, such as leaves and fish waste, using foam or matting.
Biological Filtration: Water passes over the biological media, where beneficial bacteria break down harmful compounds from fish waste.
UVC (Ultraviolet Clarifier): An integral ultraviolet lamp that clumps together single-celled algae (green water) so the mechanical filter stage can easily remove them.

Pond Filter Comparison Table

Feature	Pond Box Filter	Pond Pressure Filter
Water Return	Gravity flow (must be installed above the water level).	Pressurised flow (can feed higher points/waterfalls).
Installation/Placement	Must be easily accessible and hidden (e.g., at the head of a waterfall).	Very flexible; can be partially buried for a discrete look.
Contact Time	Longer contact time with media due to larger volume.	Shorter contact time; relies on UVC/pressure for clarity.
Cleaning	Manual cleaning every 2–4 weeks (remove and rinse media).	Often features an easy-clean backwash function for fast, hassle-free maintenance.
Suitable Ponds	Small to medium-sized ponds, especially those prioritising biological stability.	Small to medium-sized ponds where discrete placement is key, or when feeding a feature.

Choosing the Right Filter System For Your Pond

Ultimately, the decision between a box filter and a pressure filter usually comes down to your garden's layout and how you want to install the unit.

Choose a Box Filter if you have a suitable space to hide the unit at the highest point of your system (e.g., the top of a waterfall). They are ideal if you want maximum biological performance for a fish pond and don't mind a gravity-fed return.
Choose a Pressure Filter if you need flexibility. If you want to bury the unit to keep it out of sight, or if you need to push water uphill to a water feature, a pressure filter is the correct choice. They are also the best option if you prefer a quick, "hands-off" cleaning routine via a backwash system.

A Final Expert Tip: Regardless of which type you choose, we always recommend "sizing up." If your budget allows, choose a filter rated for a pond slightly larger than yours. This extra capacity provides a safety buffer for fish growth and ensures your water remains clear during hot summer months.

Ready to get started? Browse our dedicated collections below to find the perfect match for your pond.

Shop Box Filters Shop Pressure Filters

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