FWIW, I believe that it is not clear that struvite will be insoluble in our reef systems. It is not a waste water system with high ammonia and phosphate. Only if ammonia is significantly above 0.05 ppm and phosphate above 0.1 ppm will it be insoluble.
Let's look at some real values.
The Ksp for struvite in seawater is 10-(13.08) = 8.3 x 10^(-14), which is from:
Solubility of Struvite in Seawater
http://www.the-conference.com/JConfAbs/5/449.pdf
where they also claim:
"As a result of this investigation and our experimental data about unseeded precipitation of calcium and magnesium phosphates from modified seawater solutions (Golubev et al.,1999), no magnesium (ammonium) phosphate can be inorganically precipitated in modern marine environments..."
But let's use their Ksp and see how our reef tanks match up.
The Ksp is:
[Mg++] x [NH4+] x [PO4---]
Magnesium in seawater is 1280 pppm or 0.053 M
Let's examine phosphate at 0.1 ppm = 0.1 mg/L = 1 x 10^(-6) M at pH 8.1 only 20% of it is present as PO4--- (and that number drops as the pH lowers), so the PO4--- is about 2 x 10^(-7)
Let's also examine ammonia at a fairly high 0.05 ppm, or 2.8 x 10^(-6) M and assume it is all ammonium
So the solubility product in this case is 0.053 x 2 x 10^(-7) x 2.8 x 10^(-6) = 2.9 x 10-14.
So the solubility product is lower than the Ksp in seawater, indicating that it will be soluble (and won't precipitate) under these conditions.
From your citation:
"no magnesium (ammonium) phosphate can be inorganically precipitated in modern marine environments"
and yet struvite is found produced by biofilms in the marine environment and in the beds of sulphur denitrators and as I cautioned, its presence is controversial. See my original post #29. The fact that much if not most data is the result of watse water treatment studies, does not preclude their use in the marine aquarium. Even carbon dosing has its roots in waste water treatment.
from
Sulfur To Reduce Nitrate in Fish Aquariums By J. Charles Delbeek
http://www.fishchannel.com/saltwater-aquariums/aquarium-care/sulfur-reactor.aspx
"Professor Guy Martin, a specialist in water treatment at the Engineer National School of Chemistry in Rennes, France, is credited with originating the idea of using elemental sulfur and autotrophic bacteria to eliminate nitrate, but he only applied it to fresh water treatment for drinking water. Beginning in 1991, Marc Langouet, a past student of Dr. Martin, tested the method with seawater on his home reef aquariums - a risk given that no one knew if it would have toxic effects. By the end of 1994, after three years of experiments without apparent toxicity in numerous aquariums, Langouet proposed this method to Michel Hignette, curator of the Musée des Arts Africains et Océaniens (MAAO) Aquarium in Paris. There, a pilot project was launched under his care. Since then, experiments have been done on a much bigger scale at the MAAO, as well as at the Grand Aquarium in Saint-Malo, where Langouet was technical and scientific director from June 1996 to December 1997."
The inorganic chemistry of free water is not directly applicable when it comes to the chemistry of biofilms and bacterial mats running in an anerobic environment with bacteria concentrating ions in micro-environments. The skeletons of the corals we keep are a product of bio-films which are unlikely to precipitate on their own.