[Delphinus]

If you use the crystals from littlesilvermax you don't need epsom salts.

You'll be shocked at how much Magnesium it takes to raise it just a little bit. That's why I buy the stuff in bulk from littlesilvermax; a $30 bottle of liquid additive obtained locally will typically last me one water change whereas 30lbs of powder will last months and months if not a whole year or more.

What I would do is take a tablespoon (15ml) and add it and then test, and then gradually increase the amount dosed, but don't exceed say 100 per day (more of an arbitrary number on my part, but I don't like raising params too quick). But you may find you're adding 15-20 tablespoons (ie. 300ml -ish) to get that 100.

I get lazy and just dump the crystals in the sump but yeah, you can dose your topup water or just get a half litre of RO/DI and mix it in or whatever. I find that once Magnesium is at target levels, it's easy enough to maintain via water changes (I test every batch of salt for Ca, Alk and Mg and adjust my water change water to match targets). Incidentally it may shock you how much you will have to change your amounts for every new bucket, at least it did me, so I recommend this approach to everyone: test your new saltwater and dose it manually to make it where you want it.