Bingo as they do say!
Yes, heavy metal contamination is fairly common in non-food grade/agricultural fertilizers. It is why we always check.
Very small doses of some metals are acute toxins.
I really thought there must be some kind of toxicity and particularly metal.
Big, as in 60-70% water changes every other day for a week or 10 days, with continued use of activated charcoal and extra Prime dosing.
Obviously new fertz.