Size of "green water" algae

     Green algae commonly implicated as the cause of "green water" are 
     Chlamydomonas, Carteria, Chlorogonium and Volvox.  All are flagellated 
     green algae, and all are unicelluar except for Volvox, which is 
     actually a multi-celled spherical colony.
     These unicelluar green algae are 5-10 microns in diameter, not 
     including the flagella (add an extra 10-30 microns for the flagella).  
     Volvox can get huge--up to 500 microns (half a millimeter--visible to 
     the naked eye).  
     A diatom filter claims to be able to filter out particles as small as 
     one micron, which explains why a diatom filter can clear up green 
     water in just a few hours.
     I have no idea what particle size a conventional filter can handle, 
     but I can tell you my Hagen Aquaclear foam inserts were useless 
     against green water.