Coral Compass Pro · $3/month · what's included
Two common ways to pull phosphate down: granular ferric oxide (GFO) in a reactor, or lanthanum chloride dosed into a filter sock. This calculator sizes the GFO charge for your volume and type, and for lanthanum works out the exact amount needed from stoichiometry, split so phosphate falls no faster than your chosen daily limit. Dropping phosphate too fast stresses corals, and bottoming out at zero can bring on dinoflagellates.
The math
GFO: standard 1 tbsp per 4 gal, high-capacity 1 tbsp per 8 gal, up to 2× after 4–8 weeks
LaCl₃·7H₂O (mg) = PO₄ drop (mg/L) × liters × 371.37 ÷ 94.97
Lanthanum binds phosphate one-to-one as insoluble LaPO₄, so 3.91 mg of lanthanum chloride heptahydrate removes 1 mg of phosphate. For a liquid product, enter its strength as % w/v of the heptahydrate (10% = 100 mg per mL).
Try it
Sample: 50 gal, phosphate 0.30 → 0.08 ppm.
Standard GFO: 12.5 tbsp (0.78 cups) to start. Lanthanum chloride: about 163 mg LaCl₃·7H₂O in total, split over 3 days (54 mg/day).
Sources
- Bulk Reef Supply, "Granular Ferric Oxide (GFO) Instructions" (standard 1 tbsp per 4 gal; high-capacity 1 tbsp per 8 gal; up to 2× after 4–8 weeks)
- Haghseresht et al. (2009), lanthanum-modified bentonite for phosphate removal, Applied Clay Science 46 (La³⁺ binds phosphate 1:1 as LaPO₄)
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