In all but the simplest water features, the clarity of water and the health of the plants and fish depend on a balanced ecosystem. Ponds in the wild and garden pools alike contain a complex network of checks and balances that generally maintain the pond's condition without external help. If your pond is out of balance, it will give you numerous signals: bad smells, fish dying or gasping for breath at the water surface, dark or green water, as well as stunted and diseased plants. Here's how to keep your garden pool's ecosystem in balance. USE ALL THE ELEMENTS. Plants, fish, and other pond life-in water that is well aerated and maintained-work together. All of them, in the right mix, will keep your pond in prime condition. Floating plants provide shade, cool and filter the water, and control algae. Submerged plants are also filters, and they feed fish as well as create shelter and spawning areas for them. Fish control mosquitoes (they consume the larvae) and will control the algae, too. Snails, also, have voracious appetites for algae. Stock fish and plants in appropriate proportions to each other and to pond size. Experiment until the balance is right. As a rule, a 6- by 8-foot pond can handle no more than 16 fish (3- to 5-inch), six water snails (such as great pond or ramshorn), and 15 bunches of submerged plants. KNOW YOUR WATER. Invest in a kit for water testing. They're inexpensive and a number of types are available. Test for ammonia and nitrite levels when you first fill your pond and then periodically thereafter, particularly if fish are dying. If you do have a chemical problem, a partial water change, described below, can help lower ammonia and nitrite levels. KEEP IT FILLED. Don't let the pond evaporate-a drop of an inch or more below its normal water level starts to create unhealthy concentrations of salts and minerals and exposes the liner to deteriorating UV rays. When you add water, fill the pond with just a trickle from the hose (keep it at the bottom of the pond) to allow fish and other pond life to adapt to the gradual changes in temperature and pH. Don't add more than 10 to 20 percent of the total volume at any one time or the fish could go into shock. PARTIALLY CHANGE THE WATER WHEN NEEDED. Although it's best to keep the pool filled, over time-even with refills salt and waste materials build up in the water. To freshen it, drain the pond by about 10 percent of its capacity (preferably by drawing water from the bottom where concentrations of harmful substances are highest). Then refill it. It's best to do this right before a rain so rainfall can replace at least some of the water. (Never work around your garden pond if there's a threat of lightning.) PROVIDE AERATION. Whether from a fountain or waterfall, splashing water keeps the water well oxygenated, and that's essential for supporting fish. Oxygenated water also stays fresh, warding off foul-smelling bacteria that thrive in a low-oxygen environment. If you don't have a fountain, or if it's not strong enough to aerate the water and the fish are gulping for air at the surface, add a pump with an air stone on the bottom of the pond. Water from a hose is also acceptable as long as you add it slowly with the hose placed at the bottom of the pond and add no more than 20 percent fresh water by volume. KEEP THE POND FREE OF LEAVES AND DEBRIS. Debris decomposes, and if not removed, it fouls the water. Skim leaves, fallen petals, and other floating plant matter from the bottom and surface of the pool with a net. (In a very small water feature, you can probably do this by hand.) Pinch off yellowing and dying leaves; they can turn into pollutants if left unattended. If tree leaves are a problem in autumn, put a net over the pool to catch them or make skimming the pond a daily routine. In late fall, when you remove the pump for the winter, make sure the water is free of debris before the pond ices over. KEEP IT UNDER CONTROL. If fish numbers get out of hand, give some away. Thin invasive plants regularly and divide overgrown plants so that no one element begins to take over. CONSIDER A FILTER. If the garden pool has continuing problems with debris, too much light, or excessive fish waste, consider adding a biological filter to the pool setup. PREVENT RUN-OFF. When fertilizing or applying other chemicals to the lawn and plants surrounding your garden pool, don't let the materials run off or otherwise trickle into the pool water. They can be toxic to fish, and they may promote algae growth in the water. FEED FISH PROPERLY. Feed fish only what they can devour in about 10 minutes. Don't feed them unless they are ravenous. In hot weather, don't worry if the fish don't seem to be eating much. In summer, they'll get food from other sources: insects and plants. Feeding fish too much or too often will foul the water and create a need for a larger filter.