Natural pond filters

Keeping even the smallest ponds clear and free of algae can be challenging. Biological filters, which combine mechanical and bacterial filtration, help. But introducing water plants and scavengers such as water snails and tadpoles into a pond is an easier and less expensive solution.

Water plants keep ponds shaded, protecting them from heat buildup that stimulates algae growth. They also provide oxygen for fish and consume carbon dioxide, which helps keep water clear. Snails and tadpoles are nature's garbage disposals, feeding on decaying plant material and fish waste.

Plants. Water plants should consist of oxygenating grasses (which supply the water with oxygen) and other types of aquatic plants. The small pot shown above contains water lettuce (Pistia), dwarf papyrus, a water lily, and anacharis (Egeria densa), an oxygenating grass. Anacharis and water lettuce are vigorous growers and require occasional thinning.

Scavengers. A variety of snails are sold for ponds. You can also buy tadpoles. Add mosquito fish (often free at nurseries) to keep mosquitoes at bay.

Sources. Many local nurseries sell items for water gardens. Or order them from Lilypons Water Gardens (800/999-5459 or www.lilypons.com) or Van Ness Water Gardens (800/205-2425 or www.vnwg.com).