Provide proper water circulation and filtration for the pond. A pump lifts water from the bottom of the pond and keeps water circulating and moving at all times, keeping dirt and debris displaced and ready for filtration. A filter grabs the dirt and debris in the water and removes it as the water flows through. Purchase a pump and filter appropriate for the number of gallons of water your pond holds.
Add plants to your pond to help filter out debris; add nutrients to the water and improve oxygen content in the pond. Suggested plants include water hyacinth, water lilies and water iris. Plants also add to the visual interest of the pond and allow baby fish to hide among the underwater leaves and roots until they are large enough to coexist with all the larger fish.
Attach an ultraviolet (UV) sterilizer to your pond's filtration system. This helps sterilize water as it passes through the filter, reducing the danger of disease in the fish population and controlling algae growth on the concrete surfaces in your pond.
Manually remove any leaves or floating debris on the surface of the pond with a net, and carefully rake any submerged leaves and debris from the cement bottom of the pond; then scoop them out. Always exercise caution when scooping debris from the pond, as you do not want to harm your fish.
Change 10 to 20 percent of the water in the pond each month to maintain water cleanliness. Drain the pond to remove that percentage of the water, but be careful not to lose your fish -- you may want to remove them for this process if losing them is a concern. Add clean water (don't forget to treat it with your usual water conditioners before adding it to your existing water) to fill the tank back up to the proper water level.
Clean the entire pond at least once per year. Place fish in temporary holding tanks containing water from the middle of your pond, and drain the pond. Use a water hose to spray down all the concrete surfaces of the pond, brushing and scooping away any debris. Be sure to remove any loose cement bits from the pond during cleaning. Refill the pond with clean water, treated with your usual water conditioners for fish health. Allow the water to reach the appropriate temperature (the temperature recommended for your specific fish) before returning fish to the tank. Allow fish to acclimate to the water in buckets or plastic bags for 20 minutes before releasing them into the pond.
Add biological care products to the pond, such as beneficial bacteria sold in pond supply stores. This type of product contains bacteria and enzymes that help break down any organic pond sludge such as algae, fish waste and plant waste without harming the fish and plants in the pond. This prevents grime buildup on cement pond surfaces. Add the products according to the directions on the packaging.