Dosing Lanthanum Chloride with RO water, precipitating out.

Fisherman Joe · May 10, 2021

I have tested the PO4 in the RO water and it said it was 0.07 but then a few articles says this is a false reading as the water is deionised and not pH neutral, thus effecting the reading.

Why would the LaCl precipitate out in RO water if not for PO4 content?

Randy Holmes-Farley · May 10, 2021

It is not worth bothering. That is too low be be significant relative to other sources, namely foods..

Aquarium Chemistry: Phosphate And Math: Yes You Need To Understand Both

Foods are by far the most important source of phosphate in most aquariums. In considering whether sources of phosphate other than foods are important, one must carefully look to the actual amounts involved to determine whether other sources are even worth trying to minimize.

reefs.com

Comparison of Food Sources of Phosphate to Other Sources

What about other sources of phosphate, like the “crappy” RO/DI water containing 0.05 ppm phosphate? A similar analysis will show it equally unimportant relative to foods.

Let’s assume that the aquarist in question adds 1% of the total tank volume each day with RO/DI to replace evaporation. Simple math shows that the 0.05 ppm in the RO/DI becomes 0.0005 ppm added each day to the phosphate concentration in the aquarium. That dilution step is critical, taking a scary number like 0.05 ppm down to an almost meaningless 0.0005 ppm daily addition. Since that 0.0005 ppm is 40-600 times lower than the amount added each day in foods (Table 4), it does not seem worthy of the angst many aquarists put on such measurements. That said, tap water could have as much as 5 ppm phosphate, and that value could then become a dominating source of phosphate and would be quite problematic. Purifying tap water is important for this and many other reasons.