Stop hyper focusing on trying to “ supplement “ an already nute charged soil mix. FFOF is nute dense , maybe a bit too much sometimes.
However, using FFOF and basic TAP WATER ….. will work.
That’s it.
No measuring Calmag or adding Antelope shit . It is a READY TO USE SOIL (RTU) , that’s ONLY REQUIRES WATER.
Crazy huh ?
What if you planted a houseplant in the same mix …. Do you add Calmag or other crap to it ? … no.
So …. Your plant size issue is not your medium.
It’s your lighting .