I was going to ask you the same thing =)
Normally, I change their water at the same time I change the water in my reef tank upstairs. I use the water I take out of the reef tank, and do a water change with it in the 20L the Bangaii's are in. This is almost always about 4g.
I feed the babies about 2-3 times a day, so it gets a little dirty -but nothing out of hand. About half way through the week, or whenever there is a good amount of crud on the bottom of the tank (bare bottom), I siphon out the crud into a bucket; then I wait about five minutes and let all the crud settle to the bottom of the bucket; then I slowly pour the water from the bucket back into the tank, careful to be sure I don't pour any of he crud back into the tank. I hope that makes some coherent sense, haha! But, so far it has been working, and I have had no issues.
How about yourself?