Water changes!

[Aquattro]

Originally Posted by mark

depending on you tanks demands.

This is a very important consideration. If you have a huge amount of coral growth each month, monthly changes might not be enough. If you only have moderate to slow growth, then monthly might be perfect. This is where experience and getting to know YOUR tank comes into play. What works for everyone else may or may not work for you.
I did bi-weekly changes simply because I had a day off every two weeks and I had some spare time. Doing weekly could become a chore after a few years, so play around a bit and see what the tank needs.