By Catherine Neville, B.Sc., M.Sc.
Excellent pond water quality and clarity is attainable, it just takes a bit of work. Like anything in life, however, if it is worth doing, it is better if it is done right. There is no quick fix or magic potion when it comes to proper pond maintenance. Instead, it requires an understanding of the five essential elements that contribute to water quality and clarity: circulation, aeration, filtration, beneficial bacteria, and aquatic plants.
Find the right water balance
A clean pond begins with an annual spring cleanout, which mimics Mother Nature’s spring runoff. The pond is drained with a solids handling cleanout pump and discharge hose, the rocks are pressure washed to remove string algae, and the pond is flushed to eliminate leaves and debris that have accumulated and sunk to the bottom over the course of the year. Once the discharge water runs clear, it is time to refill the pond and fire up the pump.
To achieve lasting water clarity, it is essential to have a filtration system. This typically involves a biological filter/spillway or waterfall unit at one end where cascading water will create fresh oxygen for the pond. The waterfall can also be set back to create a meandering stream that gently flows into the pond. This increases the filtration capacity of the water feature as the stream bed traps debris and the water is cleaned by the beneficial bacteria colonizing in the river wash gravel. Set at the opposite end of the pond, the skimmer protects the circulatory pump and skims the debris from the water’s surface by drawing water across the length of the pond.
Keep in mind, however, water circulation and aeration alone are not sufficient for sustained water clarity and quality. Water filtration is important and can be achieved by ensuring the pond includes some biological media, typically filter pads and other filtration means, which allows beneficial bacteria to colonize. These bacteria clean the water as it passes through the biological filter.
Beneficial bacteria metabolize decaying matter and waste into ammonia, which is converted into nitrite and finally into nitrate, which is essentially fertilizer for the pond plants. Although beneficial bacteria occurs naturally in an ecosystem pond, it is recommended the appropriate amount (based on the pond’s water volume) of commercially prepared beneficial bacteria be added on a weekly basis throughout the water gardening season to allow the bacterial colony to flourish and create positive results.
It is also important not to dismantle or clean the biological filter, regardless of how dirty it appears as this will disrupt the established beneficial bacterial colony. By doing so, it will have an adverse effect on pond water quality rather than improving it.
Pressure filters or canister filters have become a popular alternative to traditional biological filter and skimmer combinations. External pressure filters can be recessed into the ground or hidden behind plants and are paired with a solids-handling pump that can sit at the bottom of the pond, without the need for a skimmer. However, if the pond is located near trees that drop their needles or leaves, then the use of a skimmer is highly recommended.
For smaller ponds where the client does not wish to invest a lot of money into filtration equipment, a Band-Aid alternative is recommended. This involves wrapping the pump with biological filtration media which allows the water to be cleaned by colonies of beneficial bacteria growing on the filtration media before it is circulated through the pond. When paired with various aquatic plants, this can be an effective and low-cost method to improving water quality and clarity.
Mechanical and natural water filtration
When selecting a pump, it is important to choose one that is appropriately sized to the pond to ensure it will circulate the entire water volume at least once per hour. For ponds less than 3785 L (1000 gal), the recommended turnover rate is increased to one and a half to two times per hour as smaller ponds are harder to keep clean than larger ponds. The latter are better able to metabolize change than smaller ponds which are more sensitive.
In the absence of a skimmer, choose a solids handling pump that is engineered to sit on the pond floor. Be sure to account for friction loss associated with the length, diameter, and type of tubing used, as well as the measured head height, which is the distance between the surface of the water and its point of origin, e.g. a spillway or an outlet from a pressure filter.
If the pond is 11356 L (3000 gal), a pump rated for 190 litres per minute (lpm) (50 gallons per minute [gpm] cannot be used. Friction loss and head height calculations must be considered; therefore, a pump with an output of at least 250 lpm (66 gpm) would be recommended. It should also be considered that the pump will need to operate 24-7—from as early as possible in the spring until the pond water is at risk of freezing.
The importance of plants in a pond cannot be over-emphasized as they are nature’s filter. Plants are an integral part of a properly built pond’s filtration system. A pond should be constructed using shelves to allow marginal plants to be installed along the edges and a deeper 0.6- to 0.7-m (2- to 2.5-ft) shelf to accommodate lilies. It is recommended that lilies cover a quarter of the pond’s surface area, while floating oxygenators, such as water lettuce and hyacinths cover an additional quarter.
The long roots of the floating oxygenators are not only superb at absorbing nutrients from the water, but they also filter silt and debris. Submerged oxygenators, such as hornwort or parrot’s feather, release oxygen into the water and trap debris against their fronds.