thanks for your help ian !
as ian suggests it is best not to dilute the spent fix with the wash water
you want as high a concentration of fix/silver for the magnet to work.
if you are looking into a trickle tank ( iron steel wool tanks ) it can scrub
the wash water, and the post-magnet spent fixer to get the rest of the silver from it.
used with the magnet, the trickle tanks can handle 1000 to 2000 gallons of waste
and are pretty efficient ... working alone, without the magnet to pre-de-silver your waste
the tanks can handle about 400-800 of waste ( depending on the size of the cartridge / tank )