If you have a natural bloom and not real muddy water don't add gyp or alum. as it will serve no purpose and may make things worse.

http://srac.tamu.edu/tmppdfs/381285-460f...e8f089e2d7d1d3a

All the coagulants mentioned can

remove phosphorus from water.

As phosphorus is an essential

plant nutrient, it may be necessary

to fertilize the pond after treating

it for turbidity. On occasion, phytoplankton

and clay can mutually coagulate, so fertilizing to start a

phytoplankton bloom may also

clear water of suspended clay particles.