  We describe here our perception of complex systems, of how we feel the
different layers of description are important part of a correct complex system
simulation. We describe a rough models categorization between rules based and
law based, of how these categories handled the levels of descriptions or
scales. We then describe our fluid flow simulation, which combines different
fineness of grain in a mixed approach of these categories. This simulation is
built keeping in mind an ulterior use inside a more general aquatic ecosystem.
