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About The Author
Authors Notes
Your First Pond
Trees & Sunshine
Take The Plunge
Preformed Pools
Installing A Liner
Making A Raised Pool
A Koi Pond
Miniature Ponds
Adding A Waterfall
Colourful Ponds
Choosing A Pump
Solar Powered Pumps
Looking After A Pump
Pond Pipework
Installing A Fountain
Self-Contained Fountains
The Leaky Pond
Planting The Pond
A Wildlife Pond
A Bog Garden
Pond Plants
Plants Round A Pond
Choosing A Lily
Floating Plants
Water Hyacinth
Oxygenating Plants
About Fish
When To Buy Fish
Choosing Fish
Quarantining Fish
Fish Under Stress
Feeding Your Fish
Holidays & Fish
Breeding Coldwater Fish
Changing Colours Of Fish
Pond Fish
A Koi Collection
Ghost Koi
Fancy Goldfish
Coldwater Catfish
Grass Carp
Rearing Trout
Swan Mussels
Visitors To The Pond
Visiting A Koi Auction
Clubs & Societies
Caring For Fish
Testing The Water
Are You Poisoning Your Fish
Ponds & Medicines
Diseases & Parasites
Disappearing Fish
Problems With Herons
Green Ponds
Fish Pond Filters
How A Filter Works
Improving Your Filter
Ultra Violet Sterilizers
Looking After A Filter
The Pond Through The Year
Spring Cleaning
Pond Plants In Spring
Ponds In Summer
Autumn & Winter
Breaking The Ice
10 Problems
Useful Facts & Figures

Allotment Articles1.
Allotment Articles 2.

How A Filter Works

When a filter is first set running, whether it is in a tank or a fish pond it will simply be a physical filter for its first few weeks of operation. This is because it takes 8-10 weeks of constant running for a sufficiently large enough colony of nitrifying bacteria to establish itself to do the job properly of purifying the water. Adding bacteria in solution can speed up the whole process, or even adding freeze-dried bacteria will start the colony off.

Nitrifying bacteria are aerobic, that is to say they live in highly oxygenated water feeding off the ammonia in the water. That is why it is essential to keep the pump running continuously.

Nitrogen compounds are poisonous to fish as can be seen by the fact that when farmers spread fertilizers on their fields and it rains washing them into the canals and rivers, the fish die. Nitrates are to be found in fish wastes along with ammonia that is also very toxic. Inside a filter, different types of bacteria break the ammonia down into nitrite and then nitrate, (which is a less harmful and more stable compound) and then eventually into nitrogen gas. 

Ammonia-----Nitrite----Nitrate----Nitrogen gas 
     NH3         N02         N03             N2 

If a filter isnít working properly, you can soon tell without even testing the water, because where the water reenters the pond you will see white bubbles. This is ammonia being oxidised to form a white scummy solid. 
If this happens do a partial water change immediately. If in any doubt about the nitrite, or nitrate levels then simple test kits are available. It is probably worth keeping an eye on the PH levels as well because this can swing wildly if the filter isnít working properly. A PH reading of 7-8 is fine, 7 being that of neutral tap water.

As a filter matures small water creatures will also establish themselves in the filter. These will help break down the larger particles of matter into a chemical solution that will again be neutralized by the bacteria. If the filter should be turned off for any length of time, such as when the pump breaks, or for the duration of winter, then the aerobic nitrifying bacteria will die and anaerobic bacteria will flourish. These produce a poisonous waste that may kill the fish when the filter is turned back on again. So the filter should be washed thoroughly before starting it up again and it should be treated like a new filter.