Changing water weekly is pretty common, and will not hurt. In fact, you could change some daily and not hurt anything, but most of the time such frequent changes are not needed unless you are having an ammonia spike or raising really finicky fish. One of the main purposes behind the water changes is to control the build up of nitrates in your water. Hopefully you are testing your water regularly for ammonia, nitrite, and nitrate. The first 2 should always be at 0 once your tank is cycled, but the nitrate just builds and builds until you remove it (dilute actually) with a partial water change. Testing the water regularly will tell you when your nitrate is getting too high, and thus tells you when you NEED to change some water out. The absolute maximum you should let nitrates build to is 40 ppm, though I think most of us will tell you it is better to keep it below 20 ppm.
If you are not already testing your water, then I would recommend that you begin doing so...it can help you head off a lot of problems, or at least diagnose them when they happen. Don't use test strips though as they are not accurate. Use a liquid testing kit. API Freshwater Master Test Kit is a really popular one that a lot of us use.
http://www.drsfostersmith.com/Produc...&N=2004+114130