Mike - do some homework for your father-in-law about grass carp eating filamentous algae(FA). They do not like FA but will eat some of it if the grass carp are over stocked and crowded into the pond to the point of making the pond muddy. Then with algae gone and the muddy water does not grow algae because the muddy water limits light penetration and algae cannot grow. Muddy water will also reduce the amount of natural fish food the pond produces then all fish are reduced.
Consider using either blue or black dye with the algae treatment program. Some refresher dye should be added monthly to maintain the proper concentration for light limitation by the dye. Pond dye dissipates naturally by: 1. UV light fading, 2. chemical decomposition, 3. dilution, 4. evaporation. Thus the need for a little monthly refreshment.
Read through this thread from our Archives section:
http://forums.pondboss.com/ubbthreads.php?ubb=showflat&Number=92633#Post92633 
