Clams work great for nitrate reduction but you do need a few of them. Here is a link to a friends tank. He didn't use VSV or pellets or GFO... just clams. At one point he had 55 of them.
http://www.youtube.com/watch?v=ZwW0H93F_u4
btw The 5" to 6" Ultra Maxima clams will be arriving in a couple of weeks. They will be available at fine reef keeping stores across Canada.