If it ties up the phosphates what would happen to the plankton bloom? If there would be none then you would lose the base of the food chain. Might have a great swimming pond with some turtles , frogs and snakes and a bunch of starving fish.

http://www.eoh2o.com/files/Lanthanum_Toxicity-Discussion.pdf

ORTHOPHOSPHATE (PO4

3) REMOVAL USING LANTHANUM CHLORIDE HEPTAHYDRATE (LaCl2 .

7H2O)

Lanthanum chloride heptahydrate has been used for many years by various private and commercial institutions here in the United States and abroad, to remove high levels of orthophosphates from fresh and brackish water systems (Disney World and Sea World of Orlando, Florida – information provided by request only), as well as the recreational pool and spa industry for over 20 years. Industry wide application rates have been denoted that lanthanum bonds to orthophosphate at a ratio of 1:1. Simply stated, it takes 1ppm lanthanum to remove 1ppm orthophosphate from water. As dry weight, this equates to 16 pounds of 35% lanthanum chloride heptahydrate to remove 1ppm orthophosphate (PO43)
from 1-acre foot of water. This ionic attraction takes place within several seconds of contact of the lanthanum and the orthophosphate in both the water column and/or sediment. Lanthanum retains ionic bonding at pH levels between 5 and 11,
making it highly useable in most water systems for orthophosphate reduction. The lanthanum acts as a flocculent, capturing the orthophosphate and settling to the lake or stream floor, or in recirculating system, being trapped for removal by mechanical filtration. Once bonded, it removes the orthophosphate from utilization by both microbial and algal species. Unless exposed to highly acidic or alkaline waters, the ionic bond will not release. As lanthanum has an ionic bonding level comparable to sodium, lanthanum will also bond and flocculate carbonate (CO3), arsenic, selenium, and chromium. Application rates can vary due to the presence of these other minerals.

Lanthanum is widely used in Japan to remove both arsenic V and arsenic X from freshwater drinking supplies (Water Environment Research, Vol. 71, pp. 299-306,1999. S. Tokunaga, S. Yokoyama, and S.A. Wasay) meeting the Japanese Effluent and Drinking Water Standards, by treating at a 3:1 ratio of lanthanum to arsenic. Again, flocculating the arsenic for removal by rapid filtration.