Tortuga Gazette 29(6): 8, June 1993

Pond Filtration Basics

by Liz Kimmel

Size of the pond, the degree of sun and/or shade at the pond site, the amounts and types of water-plants, and the number and sizes of the animals living in the pond are all factors to take into consideration when choosing a filter. More sun exposure leads to more algae buildup; more water-plants produce more plant debris; and, of course, larger animals produce a greater bulk of waste. The key is to slightly oversize the filter in order to ensure that the pond is properly filtered. If the system fails, it's a sure indication that the filter's capacity is inadequate.

As a "rule of thumb" a filtration system should be large enough to filter one-half of the water in the pond in one hour. This means that, for example, a 300-gallon pond would need a system capable of filtering 150 gallons per hour. Internal (in-the-pond) filters are suitable for smaller ponds. Larger ponds require larger size and capacity pumps and large filter units to efficiently handle the greater volumes of water. Therefore, external (out-of-pond) filtration systems are normally used for larger ponds.

Balance is the key to everything when it comes to keeping a healthy pond. Having the right filtration system, the right amount of plants and the right amount of animal life are all very important. Equally important is the right size plumbing for the pond. If the piping is too narrow flow may be restricted and the water won't be cleansed fast enough. The system will be inadequate to the job even though, in theory, the pump is the correct size.

Wrong-sized plumbing is the single biggest problem my customers seem to have. I think you need at least 1 1/2 inch pipe for a smaller pond filtration system to work efficiently; 2-inch pipe should be used to plumb larger systems. Many people will move into a house with an existing water feature, such as a fountain, and think they can go ahead and buy a filter and create a water garden without modifying the plumbing. This is just not true! Most fountain systems were not designed for future use as pond filters. Fountains do not require as large diameter piping as a pond with plants and animals does.

We keep a variety of water-plants as well as several species of fish in the demonstration water garden at Sperling's Nursery. We use a Beckett Bio-pond internal filter, "internal" meaning that the filter unit is placed in the pond itself. It is very effective against the "pea-soup" types of algae and is easy to maintain.

We chose this system because it is oil-free. Some widely-used systems use an oil-based lubricant in the pump assembly. This oil, which the manufacturer claims is non-toxic, can leak from the pump into the water, causing an oil slick on the surface. While the oil slick may deter insects such as mosquitoes from laying eggs on the water, it isn't very attractive and I wouldn't want oil all over my plants and fish. The little fish keep the mosquito larvae under control in the demonstration pond.

The Beckett unit consists of a rectangular, plastic housing with a top grate which bolts in place. The black plastic makes it practically invisible at the bottom of the pond. Inside are the pump assembly, two layers of filter foam, and special filtering devices called "bio-balls". Bio-balls of various sizes are used in some aquarium filters. They are circular in shape and are made up of numerous tiny tubes at various angles to each other that form a large surface area.

This filter system works both mechanically and biologically. By mechanical filtration, I mean the passing of the pond-water through a series of filtration media (the foam pads and the bio-balls) which physically block the solids from returning to the pond. Biological filtration is the conversion of the toxins ammonia and nitrite (from animal waste and decaying organic matter) into relatively harmless nitrates. Friendly bacteria called nitrifiers form a colony on the filtration media and process the toxic compounds, converting them to nitrates, which are used by pond plants as food.

Water is drawn from the pond into the filter unit. It passes through the bio-balls, which trap the coarser pieces of algae and also through the foam pads which trap the pea-soup slime. Water leaves the pump through a rigid plastic tube connected to a fountain sprayer accessory or through a flexible hose connected to the outlet. The filtered water can be returned to the pond either way. I don't use the fountain sprayer because it tends to clog up with debris from all the plants and animals living in the pond. The flexible hose could be incorporated into a waterfall arrangement with a descending series of rocks. It could also be used with one of those "spitting" animal sculptures. Ours is just submerged in the pond.

Maintenance on the Beckett filter is not difficult. About once a month, I take the unit apart and rinse the two foam pads with water to clean out the collected "gunk". These pads are reusable and should be replaced when they begin to deteriorate. I don't usually find it necessary to rinse the bio-balls, although if they were plugged up I would. I try to disturb the beneficial bacteria living in the pads and on the bio-balls, which contribute to the overall health of the pond, as little as possible.

Our Bio-pond unit is designed to filter a pond of about 350 gallons. The kit retails for around $115.00. It is very reliable and hasn't given us any problems since we began using it a couple of years ago. The unit plugs into the main electrical circuit through a "ground fault circuit interrupter", a necessity for electrical devices used in or near water.

Liz Kimmel is the water-plant specialist at Sperling's Nursery in Calabasas, California, where she has developed and maintains the water-plant demonstration garden.