From the moment that you put in your pond it begins to age. You start your pond with water, a few nutrients and some aquatic life. Your pond will then start to acquire nutrients. As the nutrients in your pond increase then the amount of life that your pond can support also increases. As with all life it will then die and fall to the bottom of your pond and decay and release nutrients back into the water. If left your pond would eventually totally fill up with this decayed matter, or sludge, and you would have a bog and eventually dry land once again. Of course this would take some time but if you want to keep an attractive pond and not have a swamp in your yard you need to slow this process.
If you can reduce the amount of sludge at the bottom of your pond and encourage the ability of your pond to remove it you will be slowing down the aging process. Aerobic bacteria require oxygen to live and they work about 20 times faster than anaerobic bacteria when breaking down waste. Anaerobic bacteria do not thrive in an oxygen rich environment. If you keep your pond aerated with oxygen you will be providing the best environment for these bacteria to live. An air pump in your pond will do this. You can also introduce an additive which has these beneficial bacteria and promotes the breaking down of the sludge.
In this way you can keep your pond healthy, clear and full of water for many years to come.