DG, you say the rest of her tank mates are fine, this FB GSC is the only one acting like this? ...sometimes I forget the wisdom of Occam's Razor!
If the others are fine, even after all the tweaking you have done, then this fekkin' GSC may just be one of those hopelessly bitchy pheno's... that it's moved up another set of leaves pisses me off! This is the down side to hydro, with different strains; if one dislikes it's little watery world and the others are fine, you're stuck dealing with a malcontent-
.... OK, rereading things, I see you may have been overloading with Ca-Mg, and excess Ca seems to be potentially problematic with P availability according to the charts, including the
wheel of confusion- ...
>>> I'm not sure of the exact chemistry behind it, the "why" behind how the antagonistic chemistry of Ca and PO4 (or H-PO4) ions in solution, most potential compounds are insoluble,...
... but again with the other plants doing OK, such a lockout would affect them too, and if insoluble precipitates formed you'd see them? Since MC2.0 had more Ca/Mg in it, maybe the supp' Ca-Mg is overkill?
...that hose fitting is a standard screw-off type, but things are looking pretty crusty so you may need pliers to turn it (to the left) until it's freed up.... looks like a little bit of teflon tape was used, a good thing, so it may turn more easily...