I have not found my aquarium to be especially low in magnesium when using IO, but I have also not recently tested it. FWIW, many folks find things low because they do not maintain NSW salinity (35 ppt = 1.2064 sg).
I use limewater to maintain calcium and alkalinity, which is very inexpensive.
Baking soda and Dowflake CaCl2 are cheap ways to raise them independently.