Both are doing arithmetic correctly; they disagree about one constant. Fermaid-O contributes about 40 ppm YAN per gram per litre as a raw mass balance, but organic nitrogen is used roughly four times more efficiently than inorganic YAN, which is the basis of the TOSNA protocol. Calculators that ignore the efficiency factor recommend four to five times too much.
Example, 19 L at 13% potential: needed YAN ≈ 234 mg/L.
Raw mass balance: 234 ÷ 40 × 19 = 111 g total. Too much.
TOSNA basis (160 effective): 234 ÷ 160 × 19 = 28 g total, about 7 g per addition at 0, 24, 48 and 72 hours.
The protocol answer lands in the widely quoted 3 to 4.5 g per gallon range. Overdosing is not dangerous, but it wastes nutrient and leaves food for spoilage organisms.
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