There are several steps you should take in the fall and beginning of winter to prepare your pond for the cold months. There's no real need to change your fish's diet, but once the water temperature drops below 50 degrees, stop feeding them. Before then, use the end of fall to check and maintain your equipment, and do a thorough pond cleanout, focusing on debris like leaves or other organic matter so they aren't decaying in your water throughout the winter. Tender aquatic plants should be removed and either taken inside or simply composted - you can buy new ones in the spring. For your cold-hardy plants, you can wait for the first frost and then trim the plants back to the level of the top of the pot before moving them to rest at the deepest level of the pond.
If there's any risk of a hard freeze in your area over the winter, you'll want to remove your pump and any other equipment that could be damaged by ice. Clean your pump thoroughly, then store it in a bucket of clean water in a protected area that won't dip below freezing - a garage or storage closet are good examples. The water will protect seals and gaskets from drying out. In the pond, do install a floating de-icer at the shallow end of the pond. You can add a gentle surface aerator too, if you like. The deicer is intended only to keep a small area of the water clear of ice so that critical gas exchange can occur. The aerator can contribute to that, but you do not want it to be strong enough to mix water throughout the pond. Once all the leaves have fallen in the fall, consider removing your pond net - a pond net that captures and holds snow will eventually collapse and could cause damage.
