Article by Kensin :
Changing the tank water during Winter
You can change your tank water several ways. The method of changing the water depends on the season (like winter or spring), your location, and how good of a water heater you already have in your tank. If you do not have a heater, I highly recommend you acquire one for the cold seasons.
If it is during the late fall to early spring months, you can just discard whatever amount of water from your tank (around 25% to 40% of your aquarium) first. Then, you can either do one of the following:
1) Fill up a bucket with the approximate amount of plain cold tap water you need to refill your tank, add your de-chlorinator; I highly recommend using Seachem Prime to condition your tap water. Let the water sit for a while, could be overnight, until it reaches the approximate temperature you want.
2) You could place a small water heater in the bucket to warm up the water to your desired temperature and then refill your tank. By doing this method you will not have to wait overnight to refill your aquarium.
3) Slowly refill your aquarium with fresh cold water from your tap through your hose directly into your tank very slowly. If you do not have a heater in it (I recommend you get one), then I suggest you only add 5% of the water back to your tank, and let the power filter circulate the water a little bit. This way it would not lead to a huge sudden temperature fluctuation and your shrimps will not be faced with sudden temperature shock. After 15 minutes to half an hour later, repeat the procedure until you refill your tank completely. If you follow this procedure, please do not forget to add your decholorinator or de-choloramine each time you add water back into your tank.
4) If you have a heater in your tank already, then of course you can add 10% to 15% (or more). Just make sure your heater can heat up the cold water in no time, in addition to your filter circulating the water very well. Most people usually buy heaters that are just capable to heat the size of their tanks only so it might take longer. I use a 280 watts heater for my 20-gallon long tank and it heats up the water in less than 10 minutes (40% water changes - once per week).
Even with all of these different methods, you can adjust the above conditions by yourself depending on how cold the water is from your tap, how cold it is during certain months (as well as how cold/warm it is inside your home), and how good of a water heater you have established in your aquarium.