Doing a full pond cleaning during the colder winter months can be very stressful on your fish. However, if the pond is really dirty and full of 'muck' - then you may want to consider it because all of the decaying organic matter in the pond can cause problems if the pond ices over, and this begins to de-gas and rot.

So, I think the best solution, and what we used to do for our clients was do a partial Fall pond cleaning.

Here's how to do it:

First, get a container that will hold roughly 100 gallons or so, or up to half of your pond volume (bigger is better). Then take a pump with a hose, and pump out the relativel 'clean' water from your pond by holding the pump just beneath the water surface. Keep as much of the 'old' pond water as you can. Then, catch your fish (if possible) and place them into the holding tank of their own (clean) water.

Then you can either net out your leaves and dispose of them, along with any muck that you can get out also. Alternatively, you can then pump out the remaining water and do a thorough clean out, including vacuuming out the pond with a large wet/dry vac (this works great!).

Then refill the pond back up to the level it was at before disposing of the water, de-chlorinate the water, and adjust the pH to match that of the 'old' water in your holding tank. At this point, start pumping new water from the pond into your holding tub, and then pumping the mixture back into the pond. Do this for 15-20 minutes until the new water mixture matches that in the pond - and then pump the remaining water back into your pond while netting your fish back in as well.

But it's very important not to expose your fish to new water conditions too quickly as differences in temperature and pH can cause extreme stress to your fish, affect the immune system, and even cause shock or fish death. So always be careful when changing water.