The simple water change method is to take some water out of the tank and replace it with new saltwater
Personally, though, I never just do a water change on water change day. I consider "water change day" to be "tank maintenance day", which is done weekly on three tanks.
I'm of the school of thought that an ounce of prevention is worth a pound of cure. Doing miniumum maintenance, i.e. only changing some water, is really just setting myself up to have to deal with nuisance algae/cyanobacteria and other problems down the road, which is never any fun and seems forever to correct.
Properly caring for the fish, corals and other inverts in my tanks are of greater importance to me than how little I can get away with in terms of how much work I put into my hobby. In other words, the more you put into your tank, the greater your overall satisfaction and long term success will be.
Here is my webpage on how we maintain our BB tanks. Hope you find the text and photos useful.....
http://www.lostmymarblz.com/reeftips...ottomtanks.htm