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Ponds - Water Quality and Filtration
by Darlene Jennings
by Darlene Jennings

email: ladyandherpond@sbcglobal.net

Darlene, Also known as the Pond Lady, is President, Mid-Michigan Pond & Water Garden Club

MSU Advanced Master Gardener

June 3, 2001

The quality of water in your pond will depend upon many variables - oxygen, carbon dioxide, the amount of ammonia and its compounds, level of acidity or alkalinity (its ph), temperature, amount of minerals, sunlight it receives, life forms as well as fish and any other living matter you cannot see. Debris such as leaves, fish waste, uneaten food, and dead organisms produces nitrites as bacteria break them down. Nitrites are then converted by other bacteria to less dangerous nitrates. These are then absorbed to a certain extent by plants. If the water becomes polluted then disease and death of your fish will surely follow. In any natural pond a balance has developed between animals and plants so that the waste from your fish is broken down by the plants and helpful bacteria. This keeps the water pure and healthy. In a backyard pond we must try to recreate this balance. 
If enough growing aquatic plants are living in the pond, they will act as natural filters and maintain a healthy environment. People with ponds will have problems when they have too many fish and not enough plants. Ammonia, nitrites and nitrates start to build up and eventually poison the fish and other animals. A rule of thumb is to cover about 30% of your pond with growing aquatic plants, excluding water lilies. You must also take into consideration how many fish you have. It is advisable to allow 1 sq ft. of surface area per 1 inch of fish size. If your pond is 6ft. by 10ft. it will have a surface area of 60 sq ft. You can have 60 inches of fish. Remember that fish will grow so figure out their eventual size. 
Koi produce huge amounts of waste and they like to dig up your plants and eat them. The only solution to this problem is to protect your plants from the Koi with a screening material or use an artificial filtration system. There is a bewildering array of different types available now and more coming on the market every day. Most Koi owners uses a combined unit which has a UV sterilizer built into it. This U.V. sterilizer works by killing the single-celled algae which causes green water and makes your pond look like pea soup. 
A proper filtration system should draw water from the lower levels of the pond where the dirtiest and least oxygenated water is. This in turn will draw the cleaner and more oxygenated water from the upper levels downward. A circulation is created. This will also ensure a more stable temperature. 
Filters remove suspended matter from the water and harbor beneficial bacteria that breaks down toxic ammonia into nontoxic nitrates. There are three types of filters. 

This keeps the debris from passing through the medium. Medium is a series of mats, perforated plastic, pebbles, gravel, foam, nylon brushes or any other inert matter. These filters can be attached to the pump and put directly in the water or they can be placed outside the pond. These should be cleaned every one to seven days. 

This neutralizes pollutants (chemical compounds) dissolved in the water, thus passing through mechanical filters. The medium has large absorption surfaces which attract the compounds. 

These are typically set up outside the pond. They use a bacteria grown on the filter mats or matter in a large container to neutralize ammonia and clean the water. The bacteria are naturally occurring but most ponders buy a supply to get the filter up and running quickly. The best types pump the water to an aeration tower. Water then falls down to the bottom of the container and works its way through filters filled with beneficial bacteria. The muck is left at the bottom where it can be flushed out by opening a valve, and clear water flows out the top and back into the pond. Biological filters are larger than mechanical filters but they need to be cleaned out less often. 

Every pond is different. What works for one pondkeeper may not work for another. A very important thing to remember is in spite of a pond enthusiast’s claims; there is no one right way. Visit other ponds (pond tours are one way to see different ponds and talk to different people). Visit as many different businesses that sell pond supplies as you can. I cannot stress enough the importance of gathering as much information as you can. Join a pond club or go on a pond tour in your area and ask questions. 

Safety Tip: When you are cleaning your filters, it is advisable to wear rubber gloves. This will protect you from a potential infection if you have any breaks in the skin of your hands or fingers. 

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