Mikaila did a post about that a while back in a thread in the planted tank forum here which reassured those of us reading it.
Unfortunately, I'm too lazy to go search for it.
But I've been adding Flourish to my shrimp tank ever since with no apparent ill effects.
Actually, I've read about multiple forms of various compounds which are very different in effects: in example, apparently, there's a form of arsenic produced by various plants which is harmless, unlike the type we think of as poisonous.
It's darned hard for those of us who haven't studied chemistry to sort out out what's safe without guidance.
So many related things are lumped into a single category, with the most spectacular effects of one or some of the group attributed to them all.
So many interesting things and, for at least one of us, so little time, so little brain...