Do water changes need to be done evry week??? I idnt know this, I thought it was just every 3 weeks or so.
The answer is: it depends on two things - your water chemistry and personal preference.
The primary reason for periodic water changes in an established tank is to keep the nitrate levels at comfortable (non-dangerous) levels for fish. Nitrate is the final product of the nitrogen cycle, and it does not leave your aquarium unless you remove it with some of the water. It is far less poisonous to fish than ammonia or nitrite, but given time it will build to problematic levels, and as long as you have fish it will continuously build (the higher the bio-load, the faster it climbs). Periodic water changes reduce the nitrate concentration through dilution. If you don't want to do any more water changes than are necessary, then you must first determine what nitrate level you are comfortable with, and do a partial water change when the nitrate builds to that level. In time, you'll learn how frequently you must change to keep the nitrate at an acceptable level.
Having said that, fish just plain like clean water (don't we like clean air?) - not to mention the removal of garbage from the tank if you gravel vacuum when you water change. Fish will appreciate partial water changes as often as you are willing to do them, provided you don't *over*do it. General wisdom is that you should never change more than 30% or so at a time unless specific circumstances warrant it.
Changing 20% to 30% per month is often given as a guideline, and it's not a bad one, but it is just a general guideline. As far as water changing for the sake of nitrate control goes, this is probably sufficient for most tanks, though some will require more frequent changes. I wouldn't do water changes any farther apart than one month, for certain. But remember, smaller, more frequent water changes are better for the fish than large, once-in-a-while ones. If you are willing to do it, a weekly 10% change is better than a once-a-month 40% change. Personally, this is about what I shoot for: 10% each week. Every now and then when I'm feeling particularly industrious, I'll "sneak" in another 10% change somewhere during the week -
- but not always.
Hope that helps.
pendragon!