The Mg thing likely comes from the Reef folks and a couple of marine noxious algae, they elevate Mg to kill it off(Bryopsis).
No FW experiences...........the higher Ca, Mg etc might through salt stress, but most all FW algae are very very tolerant of salt stresses.
A good general targetting rule is that you will never need to dose more Mg, Ca, etc on down the line than you do N and K.
So if you have a max uptake of say 10ppm of N(not NO3), quite a bit, you will never need more than say 8-10ppm of Mg.
I think you will find that Mg will affect shrimp and plants before fish, some species might be sensitive, hard to say without real careful testing that is species specific.
Davis tap has 52ppm of Mg. No issues there.