It all boils down to the surface area of your tank and the height of your tank.
Let's just say it use up for one to judge. Other people's comment should be taken with a pinch of salt since they have not seen your tank in person.
For instance if the clam is onli 10cm or less from your light source (and your light has reflectors to concentrate the light into your tank only), then placing it directly under a 55W 6,500K - 10,000K PL tube is more than sufficient.