In our (Presbyterian) church, we have a stone baptismal font in which we fill with water whenever we need to baptize somebody. At other times, it's empty, more for reasons of plumbing than doctrine - apparently there's some chance it could be damaged if kept full, due to its age and design. Our minister mentioned that in the Catholic Church, this would be considered wrong, as the font is meant to be kept full at all times, with the water traditionally being replaced each year at Easter.
What is the reason for this custom of keeping the baptismal fonts full? Is it a strict requirement or just a common practice? And as a practical matter, how is the water kept clean if it has to last all year?