Clean the pond bottom - Fallen leaves from nearby trees and bushes, as well as leaves from your pond plants, can quickly accumulate on the bottom of your pond. The decaying vegetation can compromise water quality so it is important to remove as much material from the bottom of your pond as possible. Also, prune your marginal pond plants and remove floating plant material before they decay. Your skimming and vacuuming or dip netting efforts will take care of the rest.
Clean skimmers, filters, and pumps - A dirty filtration system is inefficient. It works harder and accomplishes less. Eventually, it may clog and not work at all. Take this opportunity to perform comprehensive seasonal maintenance. Clean and replace filter media as needed to make sure your pond filtration system is in top condition. In a few weeks, you will be looking to minimize the amount of time you spend dipping into frigid waters.
Do a water change - Remember, when the leaves begin to change, it's also time to change your water. By summer's end, the water can be dirty and in need of a refresh. Perform a substantial water change, up to 50%, to remove built-up contaminants and help maintain improved water conditions throughout the winter. This is best done when pond temperature is the same as source water, but no lower than 60°F to minimize fish stress.
Install Netting - Now that your pond is clean and winter ready, keep it clean. A pond net draped over your pond will prevent the majority of leaves and twigs from getting into the water. Simply unfasten the pond netting and remove fallen leaves in bulk. Repeat this process until all the leaves have dropped from the trees and the area around the pond is raked free of leaves.
Switch fish foods and gradually reduce feeding - As temperatures drop below 70°F, reduce protein content in your fishes' diet by mixing a high-quality, low-protein wheat germ food. When water temperatures drop below 60°F, feed exclusively a wheat-germ-based food in smaller quantities. Below 40°F, you should stop feeding altogether.
Move plants before the first freeze - When the average daily temperature is below 50°F or before the first hard freeze, place hardy water lilies deep in your pond. If your pond is shallow, bring the plants indoors along with any subtropical or tropical pond plants you want to keep.
Aerate - An aeration kit will maintain proper oxygen levels and help keep the pond surface open when freezing temperatures arrive. If you plan to use your aerator throughout the winter months, you need to set it up correctly so it does not harm your pond inhabitants. Avoid forcing cold air into the water by housing the aerator indoors. Do not place the diffuser (airstone) on the bottom of the pond to prevent warmer pond water from mixing with the cooler water near the surface. If you live in a northern climate, invest in a de-icer and install it once daily temperatures drop below freezing.