Pond Filtration Guide

There are many pond filters available on the market today and it can be rather confusing when it comes to choosing a filter that is suitable for your pond, in this article we will discuss external pressurised and gravity pond filters and their pros and cons. 

Pressurised pond filters are perfectly suited to small to medium sized ponds and one of the biggest advantages of a pressurised pond filter is that it can be almost completely buried in the garden! The filter can also be situated outside above or below the water level of the pond. Pressurised pond filters such as the Oase Biopress and Oase Filtoclear ranges also come with a very handy, easy to clean back wash system. This means that the filter does not have to be pulled apart to clean it, the water flow is reversed through the filter and the handle is pumped up and down which squashes the sponges and the dirty water is discharged onto the garden. The filter still needs to be pulled apart every so often to check the condition of the sponges and to give the filter a thorough clean but apart from that they are very easy to clean!

How does a pond filter work?

Biological filtration performs a process known as the nitrogen cycle. In this cycle organic pollutants that enter the pond via fish waste, decomposed plant matter or excess fish food are converted from toxic to non-toxic substances using oxygen and bacteria. If the harmful substances are not broken down by oxygen and bacteria then algae is encourage to grow and fish kills can occur. The process is the same that occurs in aquariums, just on a larger scale.

What is a pond gravity filter?

biotec101.jpgGravity pond filters come in many shapes and sizes, some have UVC's, some do not but the principle for each is the same; water is pumped in through the top of the filter, which then trickles down through the filter media and is gravity fed back into the pond. As the water flows through the filter air can enter the water causing the exchange of gases enabling the different species of bacteria to grow to help keep the pond clean. This exchange of gases as well as the ability for air to enter the water is what makes the gravity pond filter ideal for large ponds and also for ponds with koi.

Gravity pond filters need to above the water level of the pond but can be easily hidden by placing plants or rocks in front of them, they are quite easy to maintain, it is simply a matter of opening the lid and cleaning the sponges of any debris. Filters such as the Oase Biotec range come with a sludge drain and foam cleaning handle where the you just squash the foam with the handle and the dirty water is removed via the sludge drain, when the cleaning is finished the drain is closed off.

A gravity filter sits outside of the pond and is fed water by a pump where it enters the filter and flows through the filter media under gravity and returns to the pond. These filters must be positioned slightly higher than water level to enable its return under gravity. Gravity filters are the most efficient and effective because they have a larger amount of filter media and higher oxygen availability for bacteria than other styles. Gravity filters are suitable for all size ponds. Oase Biotec is an example of a pond gravity filter.

What is a pressurised filter?

pressurised pond filterA pressurised filter sits outside of the pond and is fed water by a pump where it enters the filter and flows through the filter media under pressure so can therefore be positioned above or below the pond. Pressure filters must never be run with swimming pool or irrigation ‘sump pumps’ as these pump pressures are too high and will cause the filters to burst. Pressurised filters are suitable for ponds with fish up to around 7000 litres or 15000 without. Oase Filtoclear is an example of a pond pressure filter.


Available outside Australia through Amazon (see link below).

OASE FiltoClear 4000 Pond Pressure Filter with UV Clarifier


How many hours a day does my filter have to run?

A biological filter must run 24/7 to sustain the bacteria and oxygenate the water. This is why pond pumps with low power consumption such as the Oase Aquamax are recommended for filtration systems.

I've seen pond filters that look like Oase filters, are these fine?

Oase filters are so good that low quality manufacturers are trying to imitate them. Whilst these filters may look similar they are not made with the same high quality materials nor have the scientific designs. Filters are used for a purpose and just because a filter looks the same doesn’t mean it will do the same job of keeping your pond clean. While a cheaper imitation may be tempting to buy stop and think about the cost of replacing it when it doesn’t work. Always buy the best filter you can afford, your whole pond depends on it.

I want Koi in my pond, does this make a difference to the filter I need?

Yes, Koi are a beautiful fish that can grow to a large size but also produce a lot more waste than other fish species. Koi ponds require high amounts of oxygen and larger filtration systems so take this into account when purchasing your pond equipment.


What is a Pond UV Clarifier?

UV clarifierAn Ultra Violet light (UV) is a light that emits ultra violet rays. They are used in ponds to control algae that turn water green. The algae that turns water green is microscopic and as it passes over the UV light it is killed and trapped in the filter.


International viewers, the best price we could find on Oase UV clarifiers outside Australia was through Amazon (see link below).

OASE Bitron 55C UV Clarifier



Pond Aeration

A large surface area will assist in oxygenation of the pond but ideally we recommend you supplement this with an aerator of some sort, for example the Oase AquaOxy. Additional aeration will help to maximise the concentration of dissolved oxygen which will help your fish breathe and also required to sustain the beneficial filtration bacteria in your filter. Fountains and waterfalls are another great way for injecting more oxygen into the pond and looking attractive at the same time.



 A good example of a high quality Oase pond filter can be seen in the promotional video below.