Net water, not tank rating. Use the Tank Volume calculator if you have not measured it.
Magnesium chloride, Epsom salt, or a blend of both, sized to a target in ppm.
Raises 284 L by 100 ppm magnesium using Blend, 5 parts MgCl₂·6H₂O to 1 part Epsom. Magnesium moves slowly and forgives large corrections better than calcium or alkalinity, but dissolve it in tank water first and add it over a few hours.
dose = (1,350 − 1,250 ppm) × 283.9 L ÷ 116.07 = 245 g
Magnesium chloride hexahydrate MgCl₂·6H₂O has a molar mass of 203.295 g/mol carrying 24.305 of magnesium, a mass fraction of 0.1196. One gram in one litre adds 119.6 ppm.
Epsom salt MgSO₄·7H₂O weighs 246.466 g/mol for the same one magnesium, a fraction of 0.0986, so one gram in one litre adds 98.6 ppm.
Dosing only magnesium chloride adds chloride without sulfate, and dosing only Epsom salt does the reverse. Natural seawater holds both. Blending five parts magnesium chloride hexahydrate to one part Epsom salt by weight keeps the sulfate ratio close to seawater, which is why that blend is the usual recommendation. It works out to 116.1 ppm magnesium per gram per litre.
Magnesium matters because it interferes with calcium carbonate crystal growth. Below roughly 1250 ppm, calcium and alkalinity start precipitating out of solution on their own and no amount of dosing holds them steady. Low magnesium is the usual explanation when alkalinity will not stay put.
Corals consume magnesium slowly, so a well-mixed tank on regular water changes often needs no magnesium dosing at all. Test before assuming it is low.
Magnesium test kits have wide error bars. Two kits on the same water can differ by 100 ppm. Test twice before making a large correction.
Sources: Seachem Reef Advantage Magnesium · Red Sea Foundation manual (PDF) · Holmes-Farley, DIY two part (Reefkeeping 2006) · Dieck, Reef Chemistry Calculator