Yes but before the AlgaeFix was applied. So I would think with two big water changes after dosing AlgaeFix would've removed most of it.
I'm kind of leaning towards Aquasoil being added that it is changing the chemistry of the water.
Well Algaefix and/or excel,, are reported to have negative effect on a few species of plant's.And although you performed largish water changes since,,,the water changes Would in my view,, only remove what was left in the water/tank,, not that which leaves of plant's may have absorbed.
Just my two cent's.