Issues such as climate,  exposure (ie is the pond in the shade or partial/full sun),  the level of fish stock,  depth of pond and the extent of planting in and around the pond all should be taken into account to ensure that the filtration system has the capacity to deal properly with the pollutants,  toxins,  debris,  algae and other pathogens which may otherwise accumulate and develop in the pond.

Even the plumbing design is important.   Use the largest diameter pipe available and avoid the use of  "Ts" and "elbows" wherever possible.   Small diameter pipe or tubing and T or Elbow fittings cause major reductions in water flow through a filtration system.   The best plumbing to employ is black,  ribbed,  flexible hose.   It doesn't kink but does allow smooth changes of direction where necessary.

The maximum pond volumes listed above are a guide only.   They are generally based on the most favourable conditions,  including little or no fish stock.   At the other end of the scale (say,  relatively high fish stocks in a small,  exposed pond) the capacity of a pond suitable for treatment may be as little as 20% of the maximum volumes listed,  particularly in the case of some budget filters.

Another important point that should not be overlooked is the quality and functionality of the elements making up the filtration system.   Issues such as reliability,  longevity,  efficiency,  maintenance duty cycle,  ease of maintenance and overall effectiveness of individual products will all have an impact on the initial and ongoing costs associated with the purchase,  operation and maintenance of the filtration system.   As a general rule,  the higher the initial cost of a system the lower the ongoing costs and maintenance requirements.   Also,  the more effective the system will be at maintaining its functionality over the long term (say 5 to 10 years).

Ponds stocked with fish require extra attention.   It is important to ensure that the water temperature and pH of the pond do not fluctuate quickly nor do such fluctuations exceed the ranges suited to the fish.   Appropriate temperatures vary with fish species.   Make sure you get this information from you aquarium supplier when you purchase your fish stock.

Pond water pH must be maintained within the range 6.8 to 8.2 (and ideally held within the range 7.2 to 7.8).   The capacity of a pond to maintain a suitable pH balance is influenced by the hardness of the pond water - which should be at least 90 ppm (90 mg per litre) at all times.

Pond water test kits,  along with natural,  fish friendly products are available to deal with pH,  hardness and other water quality issues which can impact on the health of your pond.   Click here for more information about natural water care products

1. An effective Filter to remove excess toxins,  sludge and debris from the pond.   Units incorporating both Biological and Mechanical Filtration are necessary to achieve this.
2. An Ultra Violet Clarifier  -  to eliminate waterborne algae and pathogens and to help clump dead algae so they can be filtered from the water
3. A properly designed Filtration Pump,  capable of continuous (24 hrs/day) operation,  to circulate water and deliver pollutants and debris to the filter.
4. A means of "Oxygenating" the pond water.   Adequate oxygenation of the water is necessary to keep pond dwellers healthy and to maintain the micro-organisms which produce biological filtration.

1.   Biological Filters perform the first of these tasks.   The extent to which they do this,  and their overall effectiveness,  depends upon their design.   In an ideal world:

• Mechanical filtration will remove all unwanted debris (including bulky debris such as small twigs and leaves).   Only the better quality Gravity or Flow Through filters will achieve this effectively.
• Biological filtration will convert toxic ammonia and ammonium,  via nitrite and then nitrate,  to harmless nitrogen gas.   Each of these processes requires a different set of conditions and bacteria type.   Multi zone filters are necessary to enable the complete conversion process to take place.
• The filter will aid the oxygenation process.   Flow Through filters are designed to enrich water with oxygen as it flows through the filter.   Most simple gravity filters and all pressure filters do not aid this process to any great extent.  

• Must maintain translucent surfaces so the UV light can reach the algae and pathogens which it is to treat.
• Water flow rates must be limited so that sufficient exposure time is achieved for the UVC to do its job.
• The UV tubes must also be working at a reasonable level of efficiency.

Because of limited operational life,  all UV tubes should be replaced every 12 months.   The optimum time to do this in Australia is mid to late Winter.

3.   Filtration Pumps should meet all of the following specifications:

• Flow rate must be such as to turn over the full capacity of the pond every one to four hours (depending on fish stock levels) - possibly even more often for small,  shallow ponds with high fish stock levels.   When determining flow rate,  don't forget to take account of losses within the plumbing;  Flow reduction due to any "Lift" required;  and the effect of the filters themselves (allow for a 20% reduction in flow rate due to the filter).
• Pumps must be capable of passing solids to the filter.   To achieve this a filtration pump must be designed to allow solids to pass through without doing any damage.   This may appear to be self evident,  but most pumps are not designed in this way.   For safe,  reliable filtration systems use pumps designated by the manufacturer as filtration pumps or "dirty water" pumps (but not sump pumps as these are not designed for continuous operation).
• To provide a constant supply of oxygen to the bacteria responsible for biological filtration,  filtration pumps must be reliable and designed to run continuously for a very long time.
• Because they run continuously,  filtration pumps should also be highly efficient and require minimal maintenance to keep their operating costs down.
• Finally,  filtration pumps should run silently.   The sound of a noisy pump can destroy the ambience of a peaceful pond setting.

4.   Often,  the need for adequate levels of  Oxygen in pond water is overlooked.   However,  oxygen is vital for the survival of fish and other water creatures.   It is also required to maintain micro-organisms in a Biological Filter.   Several options exist to achieve this:

• Oxygenating plants growing in the pond
• The use of a Flow Through filter
• Including a water fall or stream in the water recirculation path
• Adding a fountain to the pond