It is one of MANY methods for reducing phosphate. I would research more of them before picking one. Lanthanum has drawbacks (such as depleting alkalinity) as well as benefits.
I discuss and compare them here:
Phosphate In The Reef Aquarium
https://www.reef2reef.com/blog/?p=3184
from it:
Soluble Metals to Bind Phosphate
There are several approaches that add soluble metals to bind and precipitate phosphate. The most popular involves adding lanthanum, which precipitates as lanthanum phosphate and/or lanthanum carbonate (which itself may contain some lanthanum phosphate). The lanthanum approach is widely used in the pool industry to reduce phosphate, and seems to often work well in aquaria. It is also very inexpensive, using products such as Seaklear (make sure it is a pure lanthanum version as mixtures with other metals also exist). Note that this method reduces alkalinity, as removing carbonate and phosphate as a lanthanum precipitate will reduce alkalinity.
One way to use it is to drip is slowly just upstream of a particulate filter to catch and remove a substantial amount of the precipitate that is formed. One drawback to the lanthanum approach is that much of the precipitated material may escape capture and simply settle out in the system somewhere. That may not be an issue, but many aquarists do not prefer to accumulate such material. A second concern is that some people have observed
problematic reactions from aquarium inhabitants. While there are not a lot of such stories, it is enough for many aquarists to look for other options.
However, due to its low cost, this approach is especially well suited to outside of the tank operations, such as the removal of excess phosphate from phosphate-contaminated calcium carbonate rock that is later to be added to a reef aquarium.
Soluble iron has also been used in this way, but not nearly so often as lanthanum.
