Water Filtration Basics


Water filtration has become less of a mystery with the growth of the aquarium industry.  Some experts promote the use of mechanical, chemical and biological filtration.  Many filtration systems on the market today include all three components in one filter.

Pond water plants to filter pond water

Constructed filter basin filled with tropical pond plants to filter water

Mechanical filtration works by trapping suspended particles until they can be physically removed later.  Mechanical filtration is as easy as scooping excess fish food from the surface of the pond or complex as designing a settling chamber for the particles to drop out of the water.  Most pressurized filters designed for ponds and even aquariums collect the suspended particles on filter material near the top of the unit while the rest of the water passes through.  Mechanical filtration is especially important in ponds and water gardens with high fish populations, excessive feeding and excessive plant or algae growth.

Chemical filtration works by attracting particles to the surface of the material used and trapping them to that surface.  The process of attracting particles to a surface is called adsorption.  Activated charcoal and Zeolite are commonly used for chemical filtration.  Activated charcoal attracts organic or carbon based impurities as well as some chemicals like chlorine.  It can also be used to remove tannins from decaying leaves.  Zeolite is used to attract and traps ammonia.  Because aquarium systems are much smaller than most water gardens chemical filtration is more important in an aquarium.

Floating islands with water plants shade the water while keeping pond water clean and controlling algae.

Floating islands with water plants shade the water while keeping pond water clean and controlling algae.

Biological filtration works by using living organisms to clean the water.  Beneficial bacteria break down organic waste into forms that plants and animals in the environment can use.  The plants and animals temporarily remove them from the system until they die and break down.  Among the beneficial bacteria colonies are the nitrifying bacteria, Nitrosomonas and Nitrobactor.  The Nitrosomonas convert ammonia into nitrite.  The Nitrobacter are responsible for converting the nitrite into nitrate.  The nitrate is then taken in by algae and plants.  Beneficial bacteria do not just grow on filter media but they also form colonies on all of the surfaces within the pond include plant roots exposed to the water.  That slimy layer on your pond is not scum, it’s beneficial bacteria or biofilm keeping your pond clean!

The most important filtration for water gardens and goldfish ponds or koi ponds are mechanical and biological filtration.  Most water gardens and backyard ponds have a pump to move the water.  That pump will also pick up suspended particles and move them throughout your system.  Adding an area for suspended particles to be trapped, such as a skimmer or waterfall box with filter material will keep these particles from clouding up your system.  Adding materials that will provide the most surface area for bacteria to grow on will keep your pond clean.

Remember, a filter doesn’t have to be an ugly canister sitting next to your pond.  A water filtration system can also be a beautiful stream full of bright flowering plants.