If you are looking to add an expansion tank to your water heater you will first have to know which expansion tank to use. You can checkout the:Expansion Tank Buyers Guide
Learn about different expansion tanks with this buyers guide complete with easy to read information.
http://wetheadmedia.com/expansion-tank-buyers-guide/
There is also a thread on RIDGID Forum that is talking about the use of expansion tanks on water heaters and how you can properly support and install them, You might want to check that out here:
Best ways to support an expansion tank
http://www.ridgidforum.com/forum/showthread.php?t=30932
If you are not a professional and you are a homeowner, you can hire a licensed plumbing company to come out to your home and take a look at your plumbing system to see which expansion tank would be best for your water heater.
