As delightful as they are to behold, fish pollute their own pond water through natural metabolism causing nitrogen based toxins to build up. These toxins will reach lethal levels if they are not regularly removed and your fish will expire.

A bio filter is thus a must have to remove suspended solids and return the water to a clean and healthy environment for the fish.

What is a Bio Filter?

A bio filter holds sponges, brushes, plastic tubing, plastic balls, lava rock, string bags and other means of trapping solid particles, all housed in some form of ‘box’. The collective name for the contents of a bio filter housing is bio media.

The bio media create surfaces for bacteria colonies to live and thrive. The bacteria work to naturally convert waste products that are produced in the pond to less polluting materials. The sponges and brushes remove some of the solids that are circulated by the pump.

Bio filters thus work by purifying the water at the interface with the bio media and also remove solids that can otherwise make your pond water cloudy.

Choosing a Bio Filter for your Pond

If you have a small to medium pond and don’t intend on stocking it with expensive fish, then you don’t need to overspend on a bio filter. Price is not necessarily the determining factor of how good a filter is. Your choices usually will be between a gravity filter or a pressurised one.

To choose the appropriate bio filter, you should know the volume of the water in your pond to about 80% accuracy. Another consideration is the greater the mass of fish in the pond, the greater the level of pollutants they generate and the larger the filter you would need. To err on the side of caution, and if you can afford it, it’s best to get a slightly larger filter to ensure better performance.

It’s important to note that a bio filter will not clear a pond where the murky water has been caused by suspended algae unless it has help from an Ultra Violet Clarifier (UVC). Most modern bio filters have these built-in and are a good buy.

There are three kinds of bio filters that are suitable for ponds with fish.

  1. Gravity Flow Bio filters – These are low-cost bio filters and can be used in any size pond. A gravity flow system consists of open tanks typically at pond level or above. These systems give you good filtration and save electricity.
  2. Pressurised Bio filters – These can be easily hidden or located at a distance from the pond. Pressurised bio filters need an extra powerful pump head to push the water through the system. Most pressurised bio filters are combined with a UVC to ensure crystal clear water and to make installation easier.
  3. Bubble Bead Bio filters – This is also a type of pressure filter and is used for larger ponds of 100 000 litres or more, rather than your average garden pond. It is both a mechanical (removes solids) and a bio filter.

Pool Spa has friendly staff who can both provide you with top shelf Bio Filter systems and also advise you on pond maintenance to keep your fish swimming in comfort. Chat to one of our experts today. Visit us at