One of the major keys in keeping pet fish healthy is keeping a clean filter in the fish aquarium or pond. Filters help keep the water from becoming contaminated and generally keep the water clear. How often you change a fish filter depends a great deal on the size of the tank or pond, the number of fish you maintain, the size and type of filter, and the number of filters you have.
Size of the Tank or Pond
Change the filter at least once a week. If a filter is getting dirty more than once a week, consider using a larger filter, or use more than one filter. It stands to reason that the bigger tank or pond you have, the bigger the filter you need because there is more water going through the filter. However, if it's a small tank with a large number of fish, you may still need more than one filter. The key is to circulate the water enough to filter it efficiently.
The Number of Fish
If you do have to change the filter or filters more than once a week, consider enlarging the area and adjusting the filter size or reducing the number of fish. It is not true, however, that if you have fewer fish, you can get by with a smaller filter than what is necessary for the size of the tank or pond. The amount of water in the tank or pond still has to be filtered. If the filter is too small to do the job efficiently, the fish will not be healthy. You can tell when you need to upgrade a filter if the fish are getting sick, the water is dirty, and the filter stays dirty regularly.
There are many different types of filters and the kind you choose can determine how often it needs to be changed. Pond filters differ from aquarium filters somewhat because the water in a pond is generally circulated through the filter with a submersible pump. Air pumps are used with aquariums and the water is usually circulated through a filter on the outside of the aquarium.
Bio-filters are popular because they use a filter media that allows the growth of good bacteria to form a natural filter. Once they are established in the tank, you should not have to clean a bio-filter more than every two months. Care should be taken not to disturb the bacteria growth, and the bio-filter needs to be washed in tepid water, not cold.
Bio-filters are often used in conjunction with mechanical filters that use carbon to filter the water. Mechanical filters need to be cleaned more often and the carbon replaced regularly. In smaller aquariums (10 to 20 gallons), the once a week cleaning rule is probably sufficient. Add ammonia removing chips to the carbon for extra protection. Ammonia build-up is a common problem in home aquariums.
The size of filter you need is determined by the size of the tank or pond you have. A good rule of thumb is to get the next size up filter for whatever size tank you have. For instance, for a 50 gallon aquarium, instead of getting a filter for a 30 to 50 gallon tank, use a filter that is recommended for a 50 to 75 gallon tank. You will be able to put a few more fish in the tank, and you should not have to clean the filter as often.