DIY Magnesium (Randy Recipe 3A) raises magnesium by 0.4694 ppm per mL per 100 L of tank water (manufacturer data: source). Enter your tank volume, current and target magnesium below for an exact dose, split automatically if it exceeds the safe daily limit.
Open the full calculatorA 75 gallon tank (284 L) raising magnesium by 20 ppm needs (20 / 0.4694) x (284 / 100) = 121 mL of DIY Magnesium (Randy Recipe 3A).
This is the error that quietly ruins an otherwise correct calculation. A tank sold as 75 gallons does not hold 75 gallons once rock, sand and displacement are counted, and a sump adds only some of it back. Real volume usually lands 10 to 20 percent below the number on the box. Dose against the label figure and you overshoot every time, always in the same direction, which reads as "DIY Magnesium (Randy Recipe 3A) is stronger than the calculator says" when the calculator was fine and the volume was wrong.
The dose above is only as good as the current reading you put into it. Titration kits carry genuine error, and a single result that disagrees sharply with where the tank has been sitting is more often a fumbled test than a real swing. Retest before acting on a surprise, ideally with fresh reagent, because a bad number and a real problem look identical on paper and only one of them needs DIY Magnesium (Randy Recipe 3A).
Do not chase it back down with anything. Magnesium that has gone high is diluted out by ordinary water changes over the following weeks, not corrected chemically. The damage in reef chemistry comes from the speed of the change far more than from the number itself, so a slow return is the correct return. Magnesium is what keeps calcium and alkalinity in solution, so this is the parameter to check first when the other two refuse to hold.