I use to get real bad problems with algea until i started using rowa phos. I've found it an absolute godsend!
Regarding how mush you need, it all depends on current phosphate levels, tank size, marine or trops etc. There should be instructions inside the actual tub explaining exactly how to dose.
I've got a juwel260 and i put the rowa phos between the last two internal filter layers so that water filters through.
Russ