Hi, More fine material will make the medium more water retentive - the laws of Physics can't be avoided. How water retentive a medium is, can be measured by a simple experiment, see the PBS wiki page: http://pacificbulbsociety.org/pbswiki/index.php/… Instead of words, which mean different things to people, it would be possible to quote numbers for the ideal medium for species. -- David Pilling email: david@pilling.demon.co.uk web: http://www.davidpilling.net/