tive phosphate ions. Subsequently, the Phoslock-
phosphate complex remains as an insoluble sediment on
the bottom of the body of water. Phoslock has been used
successfully in small lakes in Germany, Round Pond in
Kensington Gardens, London, and Emu Lake in Austra-
lia and has been adopted by freshwater aquarists. The
Magnavore company sells a product called Pura Phoslock
for the reduction of phosphate in Koi ponds.A PRACTICAL TEST IN THE REEF AQUARIUM
All this information encouraged me to test lanthanum
as a phosphate remover in my own reef aquarium un-
der routine conditions. The test aquarium was approxi-
mately 130 gallons (500 L) net. It did
not have a refugium. At the time of
the experiments, it was being oper-
ated with a skimmer and calcium re-
actor and dosed regularly with com-
mercially available trace elements. I
did a 10 percent partial water change
each month. Because the aquarium
was covered for various reasons,
the evaporated water was occasion-
ally topped off with kalkwasser. The
very dense population of leather and
stony corals was complemented with
sea anemones. The tank had been
running for about six years. However,
it was equipped with rock that had
been in previous aquariums for years.
In this aquarium, the phosphate con-
centration rose to 0.4 mg/L (mea-
sured with a PO4 Sensitive test by JBL
and a laboratory test by Triton).First, I wondered how this creeping phosphate rise
came about. There were several possible causes. Because
the tank contained very old reef rock, extensive phos-
phate deposits could have formed and then slowly dis-
solved due to biological processes. Another possible ex-
planation could have been that the water used for partial
water changes was contaminated with phosphate, which
is added to some household drinking water to prevent
the corrosion of water pipes (Brockmann 2013). I ruled
out this possibility because I had used only reverse osmo-
sis water for some time.
The main cause was probably the fishes in the tank
and the heavy feeding. The inhabitants are an eight-year-WHAT IS LANTHANUM?
The chemical element lanthanum (symbol La, atomic number 57) belongs
to the transition metals and the light rare-earth elements. Together with the
elements scandium, yttrium, and actinium, it forms the group 3 elements
(scandium group) of the periodic system. In nature, lanthanum is found
in complex minerals such as bastnäsite and monazite, together with other
lanthanides.
Lanthanum was not discovered until 1839. The Swedish chemist C. G.
Mosander described it as lanthanum because of the difficult purification
(lanthanum is Greek for “hidden”). The industrial use of lanthanum has
grown strongly in recent years. It is used in the production of catalysts, in
combination with cobalt in permanent magnets, in the batteries of electric
vehicles, and as lanthanum oxide as a component of optical glasses. The
mean concentration of lanthanum in natural sea water is 3x10^3 μg/L (=
0.000000003 g/L) at a salinity of 35‰ (Spotte 1979).PROPERTIES OF LANTHANUM
What makes lanthanum so interesting for marine aquariums is the following
three properties:
1 Lanthanum is always present in water as a positively charged trivalent
ion (La3+).
2 La3+ has an extremely high affinity for phosphate.
3 The formed lanthanum phosphate is nearly insoluble in water.
If lanthanum and phosphate ions are present in the water, the following
chemical reaction takes place:
La3+ + PO 4 3- LaPO 4
In this reaction, lanthanum phosphate is formed as a solid precipitate that is
extremely difficult to dissolve. The solubility of a solid substance in a solvent
(in this case, water) is indicated in chemistry by the term “solubility prod-
uct.” The lower the solubility product, the more difficult the solubility is. An
example: Calcium hydroxide, as the starting substance for kalkwasser, has a
solubility product of 4× 10-6 mol^2 /L^2. It is actually quite soluble. Lanthanum
phosphate has a solubility product of 2× 10-26 mol^2 /L^2. Once the lanthanum
phosphate has formed, it is practically insoluble and will remain as an insol-
uble depot in the aquarium ground. However, it has not yet been determined
whether such lanthanum phosphate precipitates can be dissolved by bacterial
and/or algal activity (and thus pose a risk). For example, cyanobacteria dis-
solve calcium phosphate using a similar mechanism.
Lanthanum is one of many tools that can
D. KNOPbe used to control inorganic phosphate.