INTRODUCTION
All reef aquarium enthusiasts are aware of the importance of maintaining nitrate and phosphate concentrations within recommended ranges. Excess or deficiency of phosphate in aquarium water has undesirable consequences for the growth and health of corals and other invertebrates. Elevated phosphate levels inhibit coral calcification and increase the persistence of algae pests, while undetectable concentrations promote the emergence of dinoflagellates and cyanobacteria. Numerous techniques exist to keep phosphate concentration within reasonable limits, such as using GFO or organic carbon dosing, which greatly helps with dinoflagellates. Lanthanum chloride is one of these techniques and stands out for its extraordinary potency and speed, which can be a drawback depending on the type of aquarium and the situation.
LANTHANUM TOXICITY
Lanthanum has a certain level of toxicity for the organisms in our aquariums. For instance, a study investigated the toxicity of lanthanum for the embryos of the sea urchin Paracentrotus lividus (Oral et al., 2012). A concentration of 1.38 mg/L did not cause embryo mortality but resulted in developmental defects in all cases. When the concentration of lanthanum was reduced to 0.138 mg/L, only 10% of the examined embryos presented defects.
In other laboratory experiments, it has been verified that zooplankton organisms like Daphnia carinata suffer adverse effects at lanthanum doses above 0.043 mg/L and die at concentrations exceeding 0.08 mg/L. This toxicity has been used as a tool to eradicate parasitic flatworm pests in acroporas (AEFW). When lanthanum became popular as a phosphate control tool in aquariums, some hobbyists using the lanthanum observed that it caused the death of flatworms of the genera Waminoa and Convolutriloba as a side effect. Dr. Samuel Nietzer, a coral biology researcher at the University of Oldenburg, indicated that the toxic effect of lanthanum on flatworms has been known and used for a long time. Daniel Knop indicated a toxic concentration threshold in aquariums of 138 μg/L, which can be verified using an ICP test. In a study he conducted some time ago, he showed that a small amount of unreacted lanthanum always remained in the water (verified by ICP).
MECHANISM
Lanthanum ions combine with dissolved phosphate ions in the water, forming lanthanum phosphate, which precipitates as solid flocs. This removes phosphate from the aquarium water. It is important to note that if no method is implemented to export the flocs, they deposit in the substrate, rocks, or mechanical filters. This is not a major problem, as the flocs cannot dissolve at the typical pH of a reef tank unless they pass through a calcium reactor, for example, where the pH is around 6.3.
One way to avoid this effect is by using a lanthanum reactor. This is a very simple device, essentially a fluidized bed reactor filled with mechanical filtration media such as polyester wool. A medium-flow pump draws water from the sump and passes it through the device. At some point, there is a T-connection where lanthanum chloride solution is injected using a dosing pump. The flocs form inside the reactor and are trapped by the polyester wool. When the hobbyist detects the saturation of the media, it is replaced.
Due to the extraordinary potency and speed of this method, as phosphate removal is instantaneous, it must be used with great caution. A very small amount of lanthanum chloride is enough to quickly reduce the phosphate concentration in the tank. Commercial products can achieve a phosphate reduction of 0.1 mg/L in water with an application of 1 mL per 100 liters of aquarium water, which is truly impressive. There is some risk of causing too rapid and excessive a drop, which can stress corals and encourage an outbreak of dinoflagellates and cyanobacteria.
HOMEMADE SOLUTION
It is easy to prepare a homemade lanthanum chloride solution. To do this, dissolve 9 grams (~2 level teaspoons) of lanthanum chloride heptahydrate in water to make 500 mL in total. 1 mL of this solution removes 4.6 mg of phosphate in the aquarium.
RECOMMENDATIONS
Lanthanum chloride works very well in aquariums with a high external food input, such as those maintaining non-photosynthetic corals. However, it is not suitable for tanks with very low phosphate concentrations, such as ULNS (ultra-low nutrient systems). The reason is that when phosphate ions are insufficiently available, lanthanum combines with carbonate ions, producing lanthanum carbonate flocs, which can result in a drop in alkalinity depending on the dosage and aquarium size. Alkalinity also decreases after dosing because corals calcify more rapidly when less phosphate is available in the water. Therefore, it makes sense to use lanthanum to reduce the concentration to "manageable" levels and subsequently maintain it using organic carbon or resins. Always ensure not to drop below 0.03 mg/L. The risk of lanthanum is twofold: it can cause excessive reduction and, on the other hand, the speed of that reduction.
Currently, the addition of lanthanum chloride without using an external reactor is commonly practiced by many hobbyists without apparent negative effects. The risks of toxicity seem to be lower than initially thought. In any case, the debate is active within the hobbyist community. If not using a reactor, the recommendation would be to dose lanthanum just before mechanical filters such as socks, rollermats, or skimmers. In this sense, the overflow is a good location. It is advisable to use low and regular doses of the solution while monitoring phosphate concentrations. Monitoring is essential to ensure that water conditions do not change too quickly, as this always has an adverse effect on corals. Phosphate concentration reduction must be measurable and traceable, and the concentration of free lanthanum in the aquarium water should not increase too much, as its possible harmful effects, particularly on corals, anemones, and other invertebrates, are not well understood. A good routine could be to use lanthanum when phosphate concentrations rise above 0.2 mg/L.
We emphasize the need to dose in very small amounts throughout the day, the smaller, the better. If the solution is added directly to the aquarium, the flocs can cause turbidity in the water, and fish, during respiration, can absorb them through their gills, where there is a very low pH due to CO2 excretion. This can cause the flocs to dissolve, leading to lanthanum intoxication. In fact, some cases of fish fatalities have been reported following the addition of lanthanum chloride.
All reef aquarium enthusiasts are aware of the importance of maintaining nitrate and phosphate concentrations within recommended ranges. Excess or deficiency of phosphate in aquarium water has undesirable consequences for the growth and health of corals and other invertebrates. Elevated phosphate levels inhibit coral calcification and increase the persistence of algae pests, while undetectable concentrations promote the emergence of dinoflagellates and cyanobacteria. Numerous techniques exist to keep phosphate concentration within reasonable limits, such as using GFO or organic carbon dosing, which greatly helps with dinoflagellates. Lanthanum chloride is one of these techniques and stands out for its extraordinary potency and speed, which can be a drawback depending on the type of aquarium and the situation.
LANTHANUM TOXICITY
Lanthanum has a certain level of toxicity for the organisms in our aquariums. For instance, a study investigated the toxicity of lanthanum for the embryos of the sea urchin Paracentrotus lividus (Oral et al., 2012). A concentration of 1.38 mg/L did not cause embryo mortality but resulted in developmental defects in all cases. When the concentration of lanthanum was reduced to 0.138 mg/L, only 10% of the examined embryos presented defects.
In other laboratory experiments, it has been verified that zooplankton organisms like Daphnia carinata suffer adverse effects at lanthanum doses above 0.043 mg/L and die at concentrations exceeding 0.08 mg/L. This toxicity has been used as a tool to eradicate parasitic flatworm pests in acroporas (AEFW). When lanthanum became popular as a phosphate control tool in aquariums, some hobbyists using the lanthanum observed that it caused the death of flatworms of the genera Waminoa and Convolutriloba as a side effect. Dr. Samuel Nietzer, a coral biology researcher at the University of Oldenburg, indicated that the toxic effect of lanthanum on flatworms has been known and used for a long time. Daniel Knop indicated a toxic concentration threshold in aquariums of 138 μg/L, which can be verified using an ICP test. In a study he conducted some time ago, he showed that a small amount of unreacted lanthanum always remained in the water (verified by ICP).
MECHANISM
Lanthanum ions combine with dissolved phosphate ions in the water, forming lanthanum phosphate, which precipitates as solid flocs. This removes phosphate from the aquarium water. It is important to note that if no method is implemented to export the flocs, they deposit in the substrate, rocks, or mechanical filters. This is not a major problem, as the flocs cannot dissolve at the typical pH of a reef tank unless they pass through a calcium reactor, for example, where the pH is around 6.3.
One way to avoid this effect is by using a lanthanum reactor. This is a very simple device, essentially a fluidized bed reactor filled with mechanical filtration media such as polyester wool. A medium-flow pump draws water from the sump and passes it through the device. At some point, there is a T-connection where lanthanum chloride solution is injected using a dosing pump. The flocs form inside the reactor and are trapped by the polyester wool. When the hobbyist detects the saturation of the media, it is replaced.
Due to the extraordinary potency and speed of this method, as phosphate removal is instantaneous, it must be used with great caution. A very small amount of lanthanum chloride is enough to quickly reduce the phosphate concentration in the tank. Commercial products can achieve a phosphate reduction of 0.1 mg/L in water with an application of 1 mL per 100 liters of aquarium water, which is truly impressive. There is some risk of causing too rapid and excessive a drop, which can stress corals and encourage an outbreak of dinoflagellates and cyanobacteria.
HOMEMADE SOLUTION
It is easy to prepare a homemade lanthanum chloride solution. To do this, dissolve 9 grams (~2 level teaspoons) of lanthanum chloride heptahydrate in water to make 500 mL in total. 1 mL of this solution removes 4.6 mg of phosphate in the aquarium.
RECOMMENDATIONS
Lanthanum chloride works very well in aquariums with a high external food input, such as those maintaining non-photosynthetic corals. However, it is not suitable for tanks with very low phosphate concentrations, such as ULNS (ultra-low nutrient systems). The reason is that when phosphate ions are insufficiently available, lanthanum combines with carbonate ions, producing lanthanum carbonate flocs, which can result in a drop in alkalinity depending on the dosage and aquarium size. Alkalinity also decreases after dosing because corals calcify more rapidly when less phosphate is available in the water. Therefore, it makes sense to use lanthanum to reduce the concentration to "manageable" levels and subsequently maintain it using organic carbon or resins. Always ensure not to drop below 0.03 mg/L. The risk of lanthanum is twofold: it can cause excessive reduction and, on the other hand, the speed of that reduction.
Currently, the addition of lanthanum chloride without using an external reactor is commonly practiced by many hobbyists without apparent negative effects. The risks of toxicity seem to be lower than initially thought. In any case, the debate is active within the hobbyist community. If not using a reactor, the recommendation would be to dose lanthanum just before mechanical filters such as socks, rollermats, or skimmers. In this sense, the overflow is a good location. It is advisable to use low and regular doses of the solution while monitoring phosphate concentrations. Monitoring is essential to ensure that water conditions do not change too quickly, as this always has an adverse effect on corals. Phosphate concentration reduction must be measurable and traceable, and the concentration of free lanthanum in the aquarium water should not increase too much, as its possible harmful effects, particularly on corals, anemones, and other invertebrates, are not well understood. A good routine could be to use lanthanum when phosphate concentrations rise above 0.2 mg/L.
We emphasize the need to dose in very small amounts throughout the day, the smaller, the better. If the solution is added directly to the aquarium, the flocs can cause turbidity in the water, and fish, during respiration, can absorb them through their gills, where there is a very low pH due to CO2 excretion. This can cause the flocs to dissolve, leading to lanthanum intoxication. In fact, some cases of fish fatalities have been reported following the addition of lanthanum chloride.
