I have some Afghan in a 5 gallon with 1/3 super soil at the bottom (subcools recipe) & FFOF + FFLW on the top. My plant has been otherwise healthy so far, but is now starting to show some signs of rust.

I've been feeding it half tap water & half RO water (tap water here is too heavy in minerals).

I'm not sure if this is a Calcium deficiency, phosphorous deficiency or something else