Choosing the Right Pond Filter System for Your Koi Pond

With so many different types and designs of pond filters on the market, it can be difficult to work out which filter is the best choice for you and your pond.

A good filtration system is particularly important for koi keepers, as koi can be fed up to eight times a day, and produce a large amount of waste and ammonia. The beautiful colours and patterns displayed by many of the different koi varieties are a result of in-breeding methods, which consequently gives them a genetically weaker immune system, and so maintaining good water quality is essential to keeping them healthy and free from disease. 

Koi also require quite specific water conditions, including pH, water hardness, temperature, and nitrate and ammonia levels.  Although a filter will not be able to control the hardness and temperature of your pond water, it will help to control the ammonia, nitrate and pH levels.

If you are in the process of designing or setting up a koi pond, we have outlined some guidance on how to calculate what type of pond filter you need.

Why Does a Koi Pond Need a Filter?

In order to understand the need for a pond filter, we first need to recognise the role that the nitrogen cycle plays.

In a pond, uneaten food, fallen leaves from neighbouring trees, and other decomposing plant and animal matter will collect at the bottom. This matter is converted into ammonium, which ammonia oxidising bacteria called ‘nitrosomonas’ then reform into nitrites. The nitrites are then converted into nitrates by other oxidising bacteria known as ‘nitrospira’.

The nitrates are then either absorbed by your aquatic plants or algae for plant growth, or denitrifying bacteria transform it into nitrogen. Whilst the nitrogen cycle is a natural process in any pond and in many ways benefits the growth of your pond plants, if there are excess levels of ammonia and nitrites in your pond, these can be harmful to your fish. Maintaining the correct balance is key, which is why it is essential to have a pond filter to help keep your water clean and healthy.

Before selecting your koi pond filter, it is important to first understand the two different types of filtration; mechanical and biological.

Biological and Mechanical Filtration

Mechanical filtration –removing all decomposing matter from the water; including fish waste, algae, leaves and other organic waste.

Mechanical filtration is not only important to help maintain clearer water in your pond, it is vital to remove the large solid matter from the water before it enters into the biological part of your filter, otherwise it is likely to clog the media and inhibit the nitrifying bacteria from converting the ammonia and nitrites in the water.

Having the correct pump flow rate is also extremely important to the efficiency of your mechanical filtration. If your water flow rate is too fast, or if the mechanical stage of your filter is too small, the water will pass through your mechanical filter too quickly and the waste matter will not have enough time to sink and drop out of suspension.

Biological filtration – removing ammonia and nitrites from your water by converting them into nitrates.

A koi pond filter is made up of a number of chambers containing different media, which act as a home for the nitrifying bacteria. Mechanical filtration is essential to the success of biological filtration as if the biological media becomes clogged, the bacteria will be starved of oxygen and will not survive.

As an oxygen supply is so critical to the effectiveness of your filter, it is advisable to use an air pipe or place an air stone under your media, to ensure there is a sufficient air supply and help keep your biomass clean.

In terms of the filtration media itself, there is a wide range available, including:

• Bio Balls

• Ceramic Bio Rings

• Japanese Matting

• Filter Box Brushes

• High Grade Activated Carbon

• Replacement Filter Foams

For your koi pond, we would recommend that you choose a pond filter that will pass your water through both mechanical and biological filter chambers to ensure healthy water conditions for your pond. Our PFC filter range offers this, in addition to a built-in UV Light, which effectively prevents the growth of algae and eliminates harmful micro-organisms and parasites which may cause fish diseases.

Calculating the Right Pond Filter and Pump Size for Your Pond

When selecting the best filter and pump for your pond, you must consider:

The Volume of Your Pond

You can calculate your water volume by using the dimensions of your pond. For example, if you have a pond that is 3m x 2m and a depth of 1.5m, the volume is 9m³.  There are 1000 litres in 1m³ so this converts to 9000 litres.

It is recommended to select a pond filter that can process up to 150% of your pond volume, so for a 9000 litre pond, this would be up to 13,500 litres.

You will also need to ensure that your pond pump can manage this flow rate, so for a 9000 litre pond you would need a 9000 litre pump rate

Water retention time

It is important for your pond water to spend enough time processing through your pond filter – if the flow rate is too great, then the water will move through the filter too quickly, and water will be returned to your pond before it is fully treated.

You can calculate your optimum retention time but dividing your flow rate by the pump rate. For example 6750 flow rate / 9000 litre pump rate = 0.75, which equals 45 minutes.

Pond Turnover

Whilst it is important to have sufficient water retention time, the filter needs to process the water at a faster rate than the time it takes for your pond to produce pollutants, otherwise known as your pond turnover rate.

This is calculated by the amount of livestock in your pond and their feed to determine the level of ammonia that is produced.

For most koi ponds, it is recommended to turn the water over at least once every two hours, so if you had a 9000 litre pond, you could pump half the water volume through once an hour using a pump that would manage 4500 litres. You would, however, need to take into consideration the height and distance for where the pump needed to distribute the water to.

So based on our above example, our PFC-20000 pond filter and AquaEco-10000 pond pump kit would be the ideal system to purchase.

Once you have calculated the above, you will be able to select your koi pond filter with confidence and ensure that your pond provides a clean, clear and healthy environment for your fish.