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Filtering Your Pond

By: Dr Gareth Evans - Updated: 15 May 2019 | comments*Discuss
 
Fish Population Faeces Nutrients Ammonia

If you want to keep any more than one or two fish in your pond you are almost certainly going to need to filter it. In nature, fish numbers are controlled by what the pond or lake can naturally support, but in the garden fish pond, the owner decides and typical stocking levels are noticeably higher than what would be sustainable in the wild.

Although goldfish and even koi can be kept without filtration, the need to strike the balance between the waste they produce and the pond’s natural ability to deal with it means that their numbers are severely limited. Since most pond-keepers would opt to have more fish than this, filtration becomes essential to prevent the toxins released by their faeces and un-eaten food from building up to dangerous levels. Though the whole subject of pond filtration has a mystique all of its own, the truth is that with the relatively recent development of solids handling pumps coupled with advances in filter technology, the job has never been easier to do.

Biological Filtration

Something in the region of five per cent of the food eaten by fish is turned into ammonia and then excreted – and with an ammonia level as near to zero as possible being ideal for fish health, the dangers of this waste chemical accumulating in the water are clear. Biological filtration offers the most effective way to deal with this. The principle is simple and makes use of beneficial bacteria which form part of the nitrogen cycle in nature, one kind (Nitrosomonas) breaking down ammonia to produce nitrites and a second type (Nitrobacter) which subsequently turn this into nitrates. By providing them with a suitable place to live within the filter itself, their abilities can be harnessed to provide a natural clean up service and yield a nutrient which is ideal for aquatic plants to use.

Although the process occurs naturally, the limited space and heavier ammonia loads in the fish pond make extra demands on the system – so nature needs to be given a bit of a helping hand. For everything to work properly, it is essential that the filter itself offers the bacteria the very best environment to live and thrive. The key to this is the filter medium – a material designed to give a large surface area for the bacteria to colonise, with lots of spaces to enable large volumes of the food-rich water to flow through. Obviously, the more bacteria which the filter can hold, the more ammonia the system can remove – making the filter medium itself key to the overall performance. There are many types on the market, including gravel, crushed clinker, expanded clay pellets and a range of plastic materials, all of which have their own characteristics.

Installing the Filter

The choice of filter is an entirely personal one and depends on a variety of factors, such as the size of the pond, the number and size of the fish in it and its level of planting. Many different manufacturers produce ready made systems, so it is worth comparing a few of the different designs and models to see which one will be best suited to your own particular pond set-up. As a general rule, it always better to opt for a filter with slightly greater capacity than you need, rather than one that is on the small size – who knows when a tempting extra fish will come along! In making the selection, another factor which needs to be taken into account is the ease – and need for – cleaning. Filters need to be run throughout the pond’s active season, so time taken for maintenance may be an issue to consider and it is important to remember that once your filter is running well, the less interference it gets, the better.

Driving the water through the filter needs a pump and it is important to select the right one; for the best results, every 90 minutes the entire volume of water in the pond should pass through the filter, so sizing the pump correctly is an important step. Principally developed to meet the needs of bio-filters the latest generation of “solids handling pumps” – named after their ability to allow small particles through without damaging their workings – offer unparalleled levels of reliability and low maintenance. Finally there will need to be some form of water return – waterfalls or cascade features being particularly suited to use as the filter outfall, with the added benefit of providing an effective means of aeration too.

All bio-filters need a little time to become established – allowing the bacteria to colonise and grow – before they are fully effective so the temptation to rush out and stock with fish must be resisted for at least four to six weeks while the filter becomes operational. It may be frustrating to have to wait, but the time can always be put to good use deciding which beautiful specimens are going to benefit from the first-class environment you have just created.

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[Add a Comment]
Im planning to build a new raised pond and need some advice. Its gonna hold 7600 litres and koi carp but i was thinking ofbuilding a bog filter? and how big would it haveto be? Or should i buy a oase biosmart 36000? Any advice would b much appreciated
Joe - 15-May-19 @ 11:56 PM
I have one of your filters but is having difficulty in making a waterproof seal can you advise
Mal - 6-Sep-16 @ 8:25 PM
I have a pond expert UC-C pressure filter model PXEF04500 but have mislaid the manual that came with it. I need to dismantle the filter for cleaning. Especially the UV unit. Is it possible to get a replacement or an on line copy please?
phil - 24-Apr-16 @ 12:23 PM
Very interesting article, my problem is thas I have no source of mains electric. Is it possible to create a solar powered filtration system? I look forward to your comments on my problem.
charlieh - 27-May-11 @ 9:51 AM
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