If there is inflow and outflow, then you will have to be constantly adding dye to get enough of a concentration to block sunlight. If you buy or make a secchi disk, and keep the visibility to 18"-24" you will minimize weed growth without dye if the bloom is dense enough. If you haven't done so, also check alkalinity and hardness of the water.

www.hoosierpondpros.com

[Linked Image from lh6.ggpht.com]
http://www.pondboss.com/subscribe.asp?c=4
3/4 to 1 1/4 ac pond LMB, SMB, PS, BG, RES, CC, YP, Bardello BG, (RBT & Blue Tilapia - seasonal).