Do Acros use a lot of magnesium?

[Aquattro]

Adding a few acro frags shouldn't affect chemistry at all over days (or weeks really).
Mg can bind with other things, I'd guess, but no, not cyano. OR that I've ever read.
"only" 1380 is more than fine for Mg, not sure what you're trying to reach.

But measure it, test again 24hr later and see the difference, if any. Keep in mind error in hobbyist test kits can be significant.