Try to mix the solution to dose for smaller amount of water like off-the-shelf ferts.
E.g. 5 Teaspoons in 250ml of water will give you a solution that raises Mg by 1ppm for every 1ml in 10l.
Your tank is 250l. Divide by 10l gives you 25. So 25 ml of the solution will raise the Mg in your tank by 1ppm.
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