A garden pond gradually turns into a bog garden without regular maintenance, so if you want to prevent yours from looking like the set of the The Creature From the Black Lagoon, be prepared for a few maintenance tasks in summer and winter, and an overhaul every five to 10 years. Maintaining your pond helps keep oxygen levels high, fish and other pond animals thriving, and your pond healthy. Summer Maintenance While a bit of sun makes a nice alternative to typical British weather, sweltering summer days and nights are no fun for you or the creatures in your pond. When pond water warms up it loses oxygen, and the level drops as water evaporates, leaving less room for fish and other aquatic animals. Warm temperatures also speed up the growth of algae and other unsightly pond weeds. Thinning and Dividing Plants Thinning and dividing pond plants prevents them from filling the water with vegetation. Aim for keeping about 50 percent of the water surface clear of leaves. Dead and decaying vegetation should be the first to go, then excess healthy foliage. Just snip the stems where they join the rest of the plant. Water lilies usually grow in pots, and these benefit from dividing and repotting in spring. Remove the container from the water, and pull or cut apart the water lily roots. Replant half the roots in the same container, and put it back in the pond. Top up pond water when the level drops 2.5 to 5 cm (1 to 2 inches). Add rainwater if you have a rainwater barrel, or tapwater if you haven't. Rainwater is better because tapwater contains nutrients that helps algae flourish. Add water slowly to avoid shocking fish and other pond animals. Remove blanket weed and other pond weeds when they cover more than one quarter of the pond's surface. Twirl a stick through the weeds and drag them out to the side of the pond. Leave the weeds overnight to allow pond creatures to escape back into the water, and put the weeds on a compost pile or your garden waste recycling bin. In hot, humid, still weather, fit a soft, fine spray attachment to your garden hose, and gently spray the pond surface every evening to help raise the oxygen level in the water. Alternatively, install a fountain feature to keep the water moving. Winter Maintenance While you're snuggled up indoors with the central heating on, most of the creatures in your pond are also nestling down for a winter rest, but fish sometimes need a little help in freezing weather. Oxygen levels can drop dangerously low for fish if a pond freezes over. Keeping an area free of ice allows oxygen to dissolve from the air into the water. If there are fish in your pond, keep an area frost-free. When temperatures fall so low that your pond surface doesn't thaw during the day, fill a pan with hot water and put in on the ice. Place it near the edge of the pond and not in the middle, or when the heated base melts the ice, that might be the last you see of your pan. After half an hour or so, when the pan has melted through the ice, remove it. Low light levels can spell problems for pond plants, which continue photosynthesising and producing oxygen under ice. If a heavy blanket of snow covers your pond, brush it away to allow light through. Deep Clean Ponds plants and algae naturally die down and fall to the bottom of a pond, creating deeper and deeper layers of decaying vegetation and mud, which eventually fill the water. Prevent your pond from turning into a swamp by removing the mud every five years if your pond is small, or every ten years if you have a large pond. Prepare buckets, bins or other large plastic containers to hold fish and other water creatures, and siphon some pond water into them. Siphon out the rest of the pond water, and catch and remove fish and other animals as they appear. Place these and water plants in the holding containers. Scoop out the mud from the bottom of the pond, and spread it on your garden borders. Refill the pond -- with rainwater if you have it -- and return the plants. After five or six hours, when the pond water has reached the same temperature as the water in the containers, return the fish and other creatures. Tips and warnings Falling leaves can fill a pond with decaying vegetation. Remove leaves that fall into your pond in autumn. Don't fertilize a lawn near a pond. Rain washes fertilizer into pond water and feeds algae. If your pond water is permanently green, it's probably receiving too much sunlight. Grow water lilies and other floating pond plants to shade the water.