For mushroooms it's probably not as important as it for stony corals.
I have 9 differnet kinds of mushrooms and keeping the water parameters ideal will accelerate their growth, they are under 2x39W T5HO in a 29G. they say they do well in dirty water but from my experience and LFS near with pristine tanks they do even better in cleaner water and higher lighting from MH and high powered LEDs.
If you want, you can search for a guide on using epsom salt for mag supplement or buy off the shelf. I bought Kent Mag instead, the seachem stuff I have looks like refined epsom salt by looking at the ingredients. I dont' test for mag or have been dosing, I do weekly water change. My tank is mainly soft corals now, maybe when I start adding some LPS I would.
