LaCl does a good job of getting things down to .03 or so. After that, it seems it starts to negatively affect alk levels (or so they say).
Gey the sea kleer. Way cheaper. Pool place on Dorsett just easy of 270 is where I got mine.
As far as releasing back into the water column, it is said it only happens at low ph levels, lower than would ever be in a marine aquarium.
But it has been documented to have a negative effect on some fish, mostly tangs if I remember, if you don't catch all the flocculate. It irritates their gills. Can also cause a white haze on the glass that can be hard to remove.
Some people have used a brs reactor packed with floss . They feed the LaCl into the feed line of the pump.
I dilute it at a 100:1 ratio and dose 1 ml of solution every 15 minutes, into a 30 g can with a power head in it. can is probably fed 100 or so gph. It gives the LaCl time to work on 30 gallons of water instead of just what is contained in a filter sock. The can's outlet is fed through a 5 or 10 micron filter sock. Sock usually clogs every 3 or 4 days .