I've had trouble inducing Mg deficicency with reg water changes with
most tap water.

You can add dolomite if it's for Mg/Ca and KH(CO3).

MgCa(CO3)2
Get fine ground, this will dissolve better.

Most folks use baking soda for ther KH. CaCl2(pool stores often carry
it) and MgSO4(Epsom salt) for Ca and Mg.

These dissolve immediately.

Regards,
Tom Barr