Pumps and Filters for your Garden Pond - Basic considerations

Pumps        Pump Position        Mechanical Filtration        Biological Filtration        Ultra-violet Filtration        Choosing a Filter       

First filter re-build - (Page 2)        Second filter re-build - (Page 3)        Final filter re-build - (Page 4)       


Pumps and Filters

Not many gardeners are lucky enough to have a stream running through their garden. For the rest of us the best that we can hope for, is to use a pump to circulate the water from the pond. In most cases, you only need to raise the water about a metre or so and modern pumps can achieve that easily. Also, if you have fish in your pond, you will need to have a filter to help remove the fish waste from the pond.

There are two ways to drive the filter. For small ponds like mine, a pump is submerged in the pond and the water is pumped into the filter system before being allowed to flow back into the pond. For larger ponds, a better way to do it is to use a bottom drain connected to the filter system via a large diameter pipe (usually about 4" or 100mm) with the pump submerged in the last chamber of the filter to pump the water out of the filter and back to the pond.

There are advantages in both methods and the choice depends on your own circumstances. Where the water is pumped into the filter, the filter can be positioned anywhere in the garden and doesn't have to be at the same level as the pond. It is also easy to make any changes that might be needed during the life of the pond. Where the water is pumped out of the filter, the system is reliant upon gravity to maintain the flow into the filter. The work to fit the drain must be competed before the pond is filled and it is extremely unlikely that you would be able to make any changes without a complete rebuild of your pond. The water level in the filter must be exactly the same as the level of the pond.

As you can see from the diagram here, my pump is positioned in my bottom pond. The depth is about 90cm. The water is then pumped up to ground level and the pipe goes under the gravel, under the bench and into my shed. Inside the shed is my UV filter and from there another pipe runs for about 12m alongside the wall, rising slightly to get to my main filter. (That's the black box in the bottom right of the photograph. It will be covered with ivy in another year or so and will be invisible.) The outlet from the filter is fed under the earth to appear as a spring in the bottom left of the photograph. The total height that the water is raised is about 1.4m from the pump or about 50cm from the surface of the pond.

So the big question is, how big a pump do I need? If you read the text books, the usual answer is that you set your pump size to match the capacity of your pond. However, lets take that argument to the extreme and say that you have a 2000 gallon pond. Therefore, the books would suggest a pump with the ability to pump 2000 gallons per hour. Imagine that lot going over a waterfall with a width of 15cm. The sound would be completely wrong and the likelihood is that not enough water could get over the fall, resulting in an overflow from the pond above the waterfall.

I have found that as long as your filter is the correct size, and the water is continually pumped, the water will clear and get purified, even with a much lower flow rate. As a general rule, to produce a "sheet" of water over the fall, the flow needs to be above 50 gallon per hour per 2.5cm. So for a 15cm wide fall, you must have at least a 300 gallon per hour pump. Once you get to that flow rate, the rest is personal preference. However, please check the noise section in my Waterfalls pages.

Pump Position

It is important that all the water in the pond passes through the filtration system. Therefore the position of the pump needs some thought. If the pump is positioned too close to the waterfall, its quite possible that the flow of water will be over the waterfall and straight down into the pump and around the system again. The result would then be that a large volume of your water is not being filtered and in extreme cases, some areas of otherwise healthy ponds could become stagnant. So, ideally, the pump should be placed as far as possible from the waterfall as I have in my pond.

Wherever you decide to place your pump, remember that you will need to be able to lift it on a regular basis for routine maintenance, so don't simply launch it into the middle of your pond. Most modern pumps are supplied with a 10m waterproof cable so the location of a suitable mains supply needs to be considered. NEVER use the power cable to retrieve the pump from the pond. If you cannot reach the pump, use the water pips to lift it. Any connections, especially extensions, must be kept waterproof. Ideally they should be made inside a building.

I leave my pump running throughout the winter so that my stream and waterfalls are always available for the birds and insects to bathe and drink. For that reason, I have to move the pump once the frosty nights arrive. Generally, the pump needs to be at the deepest part of the pond so that it picks up all the debris from the bottom and transfers it to the filter. However, fish need to be protected from extremely cold water. If you switch the pump off, unless your pond is very small, the temperature at the bottom of the pond would not drop below 4 degrees. However, if your pump is running and its pumping that relatively warm water over ice at the edge of your ponds, the water temperature at the bottom of the pond could drop below 4 degrees, and your fish could die. Don't worry though, the simple action of moving the pump to a shallower depth will solve the problem. I move my pump up onto a shelf in the middle of my pond so that it is about 30cm from the bottom and I have never had a problem, even with my ponds so covered with snow and ice that only one of my waterfalls was not frozen. Top

Pumps and Filters

If a garden pond is big enough and it has the perfect balance between the number of fish in it and the amount of plants that are growing in it, the water would be perfectly clear and the fish, plants and the rest of the wild-life supported by the pond would remain healthy. That is because the plants absorb all the waste products that the fish produce. Unfortunately, it takes many years to achieve that balance, and it can only be achieved by keeping the number and size of the fish quite small.

However, the majority of us want to be able to see a good number of nice big fish swimming happily in a pond that is, theoretically, far too small to support them. In order to achieve the bigger fish, we feed them additional food over and above that which the pond would provide naturally, thereby causing them to produce more waste products than the plants can absorb. Without help from us, the waste products will eventually poison the water and everything in the pond will die.

So because we want to upset the balance of the pond by having bigger fish, we have to redress that balance, and the way we do that is through filtration. There are three types of filtration to consider and they all play their part in keeping the fish and the pond healthy. The filters types are: Top

Mechanical Filtration

As the name suggests, mechanical filtration is achieved through physical means. ie the water is pumped through a mesh of some sort which is designed to catch any particles of dirt, uneaten food pellets, algae and any other foreign matter that happen to be supported by the water. Obviously, the finer the mesh, the smaller the particles that will be captured. It is usual to have more than one layer and to have each layer progressively finer. In that way, each layer of the filter has its own job to do and it avoids having small particles passing through a large mesh, or large particles blocking a fine mesh.

Because of its physical nature, the mechanical filter begins to clean the water immediately it is switched on. It doesn't need to be in use all the time. For example, if the pump was not running, the filter can be left unattended for a couple of days. If the filter is not required for prolonged periods, the filter meshes should be rinsed and the filter cleaned to avoid a build up of gases from the decaying matter that will have collected in it. Top

Biological Filtration

Whilst a mesh filter can remove virtually all the solids from the pond, it cannot remove all the microscopic particles of fish excreta and dead organic matter. Nor can it remove the ammonia (a byproduct of fish urine) which is harmful to the fish. The more fish you have and the bigger the fish, the worse the problem. Koi produce 6 times as much urine as goldfish and are much more susceptable to illnes due to poor quality water.

For this purpose, a biological filter is required. Quite simply, this is a chamber full of material that provides a suitable environment for bacteria and other tiny organisms to flourish. The greater the available surface area, the bigger the colony of bacteria will be and the more effective the filter will be. As the water passes over them, they extract all the impurities, rendering any remaining material harmless to the fish.

Important considerations with this type of filter are that the water has to move past the bacteria sufficiently slowly for them to have the opportunity to carry out their work. So a balance must be struck between the optimum flow for your filter and the desired effect of your waterfalls. It takes a number of weeks for the bacteria colony to grow to sufficient numbers where they can begin to effect the purity of the water. The bacteria need a constant source of nutrient from the pond in order to survive. Therefore, this type of filter must be kept running for all but the briefest periods. If the pump is switched off for a few hours, the filter will take a few days to recover to full performance, but if the pump is switched off for more than a day, most of the bacteria will die and the filter should be emptied and cleaned to avoid contamination of the pond. The filter will take several weeks to return to full efficiency.   Top

Ultra Violet Filtration

Even the biological filter can't remove all the algae in the pond and until recent years, it was not unusual to see garden ponds where the water appeared to be quite green. Although that state is perfectly OK for the fish, there is no point in having a pond where you can't see your fish. As the other methods of filtration got the water clearer, so the sun was better able to penetrate the water and provide sufficient light for the algae to flourish. The best method to avoid that is to completely cover the pond so that no sunlight can get into the water. But that rather spoils the whole idea of having a pond in the first place. The modern answer in an Ultra Violet filter. This is usually a cylindrically shaped chamber which has a clear, hollow tube within it. The water from the pond is pumped through the chamber to pass around the hollow tube. Within that tube sits an ultra violet light. The light kills the algae within the water. Once the algae is dead, it is no longer able to swim through the filter and it is eventually caught by the mechanical filter.

Like the biological filter, the ultra violet filter needs the water to be pumped through it continually as it relies on the water to cool the bulb. If the pump stops, the light will quickly burn out. Always ensure that you switch off the UV filter before you switch off the pump. Likewise, make sure that when switching the pump back on, the pump is operating correctly with full water flow before switching the UV filter back on.  Top

Choosing a Filter

I'll start with the ultra violet filter. Here the choice is simple. The filter should as a minimum, match the volume of water in your pond. These are specialist products provided by the water garden centres. If you are luck,you might get away with a slightly smaller one, but if you want clear water you need to get the right one for the size of your pond or bigger. If you intend to keep Koi, you don't want to cut corners here. If you are going to spend a lot of money on your fish, you need to be able to see them properly.

Next, the mechanical and biological filters. These are usually combined into the same box and a good water garden centre will have a large range of filters to choose from. Some of the smallest filters resemble a small barrel and are sealed so that the water under pressure from the pump is forced through the filter and then back into the pond system. These filters are quite efficient and often incorporate an ultra violet filter within the casing. However, they need to be cleaned regularly to prevent them from blocking up.

Non pressurised filter boxes are designed to have the water spray down onto some form of mechanical filter before passing into another chamber for biological filtration before being allowed to flow back into the pond system. Most filter boxes perform the job of mechanical and biological filtering by having at least one chamber for each function within the box. (It is even possible to get filters that incorporate an ultra violet filter as well.) The choice of filters available at the water garden centres is bewildering, as they come in all shapes and sizes. Just remember that all we are trying to do is to physically block the solids and provide a home for bacteria to eliminate the rest.

Within reason, the amount you spend on your filter is purely personal choice. I have seen filters at my local water garden centre that cost over 1,000 and filters for specialist Koi ponds can run to tens of thousands of pounds, and I have no doubt that they are superb at their job. However, for most garden ponds, there is no need to spend that sort of money and filters can be obtained for much less. If you want to buy one, my advice is to go to a reputable water garden centre and seek their advice.

Whatever you decide to do, the physical size of your filter needs to be borne in mind when planning your pond design. The barrel type filters are usually less than 50 cm in diameter and can be buried to the level of the lid. Some box filters can be buried to within about 20 cm of the top but some of them have outlets from the base and therefore need to be raised above the highest point in your pond system. As some of these filters are about 1m in height, you then have to consider how you are going to disguise the box so that it doesn't spoil you view of your garden.

If you feel adventurous and you could always do as I did and build your own. Its not difficult and enables you to spread the cost by buying the components over a period of time. I have always built my own filters and the final version easily copes with my pond system throughout the year and needs only to be cleaned out a couple of times during the winter and about every fortnight through the summer months.  Top

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