I will take everyone’s recommendation and run an experiment this weekend. I have all the test kits, PH, hardness, phosphate, etc. I also have the Lanthanum Chloride. I will check the pond water before adding the flocculate agent, then slowly add to say one liter a drop at a time. I will add until the phosphate levels get under 0.1 ppm. I understand that's the magic number where algae growth should substantially slow down. I will also check PH and hardness.

From this I can extrapolate the amount needed to treat the whole pond. I have one surface acre approximately 6-7 average depth. I can then determine the cost benefit trade off.

One concern I have based on the above post is the Lanthanum will also flocculate with carbonate. I know I have hard water, so the carbonate may consume all the Lanthanum Chloride before it has a chance to tie up with the phosphate. This would require way too much to be cost effective.

I'll let the forum know.