Providing adequate filtration for a pond or water garden can be an uncertain proposition when we’re still in the planning stages. Decomposition, algae growth and excessive nutrients can cloud the water, marring the beauty of our ponds, and also endanger the life of fish by subjecting them to parasites and toxic gases like methane and hydrogen sulfite. The methods of filtration that we use don’t need to be complicated, however. Locating our ponds in areas that are slightly elevated above the surrounding land, to prevent runoff contamination, and away from trees and shrubs that can drop leaves into the water will alleviate some of our potential maintenance problems. In addition, we can create a system wherein nature performs the clean-up duties if we stock our ponds with submerged aquatic plants.
Natural filtration is a biological process, the basic aim of which is to clear out or use up the minerals, phosphates and other excess nutrients that green-water algae like to feed on. Submerged aquatics fulfill this function by absorbing (and thus removing) these nutrients themselves. The basic rule of thumb for this natural filtering system is to use 5 or 6 plants in a bunch for every 1-2 square feet of water surface. More might be required if the pond is heavily stocked with fish.
Some aquatic plants that make great natural filters include water hyacinth, watercress, cattails, papyrus, water iris, arrowhead and water lettuce. Keep in mind that tropical submerged plants won’t survive cold winters, and hardy plants adapted to cold climates won’t grow as well in the warmer waters of a tropical pond. If we’re uncertain about the fitness of certain plants for the areas we live in, it might be helpful to seek the advice of knowledgeable people at a local nursery before buying them.
An alternative method of natural filtration involves using a separate basin, about a tenth of the size of the main pond, which is filled with aquatic plants. Pond water is then recycled through this basic every 2 to 4 hours. The plants will remove the excess nutrients that feed algae plant forms and also filter out other forms of particle waste in the water. If the basin is 12-18 inches deep, this will allow solid waste to settle better. To maximize settling, the hose through which the water is pumped should run from the farther ends of both the main pond and the basin.
Using submerged aquatic plants in our ponds not only simplifies the filtration process but also provides us with another spectacle of beauty within our landscapes. In this way we mimic nature’s own processes, which have been time-tested to an extent that no man-made systems can ever be.