I like to keep some sediment in the bottom of my pond for my crawfish, so I dont have the need to drain the water and clean it out yet. I think water changes should only be done for emergencies in the case of your water ph levels putting the fish in danger. I dont have much pond experience, but did experience the same problem with my first pond. Now that Ive gone larger, put in the proper pumps and filtration, Im proud to say ponding has never been easier. Plants are the best solution for keeping a healthy balanced pond. There are a lot of plants that you can plant in and around the waters edge. Cali lilies grow half in the water and half in the dirt in my pond, same with elephant ear and other beautiful large leafed plants. Hiacyinth will spread quickly sucking up a lot of the nitrogen and putting oxygen back into the water. Frog bit is another plant that will spread quickly, http://hoskingnursery.com/library/frogbit.jpg. I have so many plants in my pond that I have to sell them to nurseries and pet shops, boo hoo.