Winterizing Your Water Garden
Autumn is the time to put your water garden to bed and decide what to do with your fish. As the water temperature in your garden pond cools down, you'll cease fertilizing, and the plants will begin to go dormant. Lower the water level a few inches, and float a ball or block of wood on the surface to prevent a total freeze-over. (A complete coating of ice is bad because it prevents the exchange of oxygen and allows toxic gases to build up. Fish and plants below, even when dormant, can perish.)
Tropical plants should be hauled out of the pond and tossed onto the compost pile. Or if you want to overwinter them indoors, get the details from wherever you bought the plants or try to find a more experienced water gardener to help you. Some tropicals can stay in heated aquariums; you can strip others of all growth and store their little tubers or rhizomes in damp sand.
Hardy plants should be hauled out, too, and given a haircut, leaving only a stub of foliage. If your winters aren't too severe, (Zone 6 or warmer) you can return them to the deepest part of the water garden (no elevating supports now) for the coming months. Otherwise, they can come indoors to a non-freezing place, heaped with straw or another blanketing of mulch until spring returns.
Many pond fish can remain outside during the winter, but it really depends on where you live and on the type of water garden you have. Depending on these conditions, you may have to set up an aquarium inside your home and transfer your pond fish there over the winter season. Again, get help and information from wherever you buy your fish.
Fish that remain in the pond slow down and go dormant in cold weather, just like hardy plants. Reduce feeding in the fall. Eventually, they'll retreat to the deepest part of the pool, perhaps burrowing into some muck there. If you fear for their survival, you can net them and keep them in an aquarium for the winter.