• Pond Filtration


    • Increasingly essential to successful pond keeping, a filter system can take many forms from simple pump pre filter (in pool) units to multi chambered external filters with Ultraviolet units. Also available are in line magnets and descaler units.

    The purpose of a filter system is usually two fold:-

    1. To provide clear water.
    2. To provide a safe water quality for the fish.

    (The latter being the more important.)

    Filter systems usually therefore have two or more stages - a mechanical filter to trap the suspended particles which are causing the water to be unclear & a biological stage which houses nitrifying bacteria which break down the dissolved organic waste created by fish, decaying vegetation and surplus food. To do this, the bacteria require a constant flow of oxygen so the filter system (and pump) must run 24 hours per day. The bacterial colony can take up to six weeks to establish completely so the full benefit of the filter is not likely to seen for a couple of months.

    The Nitrogen Cycle

    Toxic Ammonia is produced in a pond by fish, decaying leaves from trees as well as water plants, surplus (uneaten) fish food & dead fish or other animals (which may have drowned in the pond). This Ammonia is dissolved in the water and is completely colourless. The first group of Nitrifying bacteria destroy Ammonia with the aid of Oxygen and convert the Ammonia into Nitrite. The Nitrite is also toxic to the fish, but a second group of Nitrifying bacteria occur and destroy the Nitrite (again using Oxygen) and make Nitrate which is considerably safer for the fish (extremely high concentrations are to be avoided). The Nitrate is a fertiliser and feeds the water plants OR algae (insufficient planting of the pond will result in more Nitrate being available to the algae and thus more growth of either "Green water" or "Blanket weed").

    Note: Ammonia is more toxic under certain combinations of  pH and water temperature. We would advise regular testing of the water for Ammonia, Nitrite, Nitrate, pH and Temperature to give the full picture on the state of the water conditions.

    There are many factors that affect the choice of filter and therefore filtration is not a precise science!

    However some basic rules do apply:-

    1. The pond water should be passed through the filter and ultraviolet unit at the rate of half of the pond per hour (the entire pond volume every two hours)
    2. Ultraviolet units have a minimum flow rate as well as a maximum flow rate. This is to ensure that the water is exposed to the UV rays for a sufficient time for the rays to be effective. The free floating "Green water" algae cells can double every four hours under ideal conditions so if the water is not treated often enough, the algae replenishes itself as fast as it is being killed.
    3. Maintenance of the filter should be practical to suit your lifestyle.
    4. If the outflow from a filter is to be utilised to flow down a waterfall, the desired effect may require a faster flow that the filtration turnover rate.

    WARNING

    If fitting an Ultraviolet Unit to an already "Green water" pond, BEWARE as the resulting dead algae cells will decompose rapidly and release vast levels of toxic Ammonia into the pond water. The biological stage of even a mature filter is unlikely to cope with the sudden additional load and the Ammonia will peak too high and cause distress or even death to the fish. A new filter system or a newly cleaned old system or an old system with new filter media (such as new foams) will all have very few nitrifying bacteria present at all so the problem is even more acute. We strongly advise the use of test kits to monitor the water during the maturing stages of any filter system - usually the first two months.

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