If you have a tank set up using the Walstad method, there really is no need to do water changes unless there is a problem like a sudden illness or if the plants an/or fish don't look happy. The plants use the fishie poop and uneaten foods as fertilizer.
If you ever decide to set up any tank like this, I strongly suggest you read Diana Walstad's book first so you understand everything because there is a lot you need to know about this type of set up and it's best to educate yourself first before just diving right in.
Both of my tanks are set up this way and I love it because you can enjoy keeping aquariums while conserving water!
