Sometimes phosphate can be so stubbornly low that the only way to meaningfully raise it is by directly dosing phosphate.
When you feed foods, you’re adding nitrogen, carbon, and loads of organics. This effect is compounded if you’re trying to offset rocks that initially work against you by binding up most of the PO₄ until a higher equilibrium is met. It’s much cleaner and more cost-effective to
purchase pure sodium phosphate online than to use hobby-grade products with unknown purities and byproducts.
You can mix sodium phosphate with nitrate, ammonium, or even add it to an ATO; however, phosphate should
not be mixed with limewater (kalkwasser), as it will completely precipitate as calcium phosphate. You may need to dose more than expected, as calcareous sand and rock can bind much of it, especially when phosphate levels are near zero.
Sodium Phosphate Dibasic (Na₂HPO₄) Recipe:
Sodium phosphate dibasic (Na₂HPO₄) is 66.9% phosphate (PO₄³⁻) by weight.
0.02 ppm in 100 L = 2 mg of PO₄³⁻.
2 ÷ 0.669 = 2.99 mg of Na₂HPO₄.
Dissolve 3 g of Sodium Phosphate Dibasic (Na₂HPO₄) in 1 L of freshwater.
Dose 1 mL per 100 L to raise PO₄ by 0.02 ppm.
Note: Monobasic, dibasic, and tribasic forms are interchangeable. Concentrations vary slightly but are functionally similar when accounting for rock adsorption.
If calculators are easier, you can follow this link. Select “potassium phosphate.”
http://www.theplantedtank.co.uk/calculator.htm
