i had that happen when i added ferts which had copper.
i did an 80 % water change (added new water slowwwwly) added some charcoal (but probably didn't need to since I changed so much water)
and I double dosed my deCHLORAMINATOR (I have Chloramines, not Chlorine in my city water)
that solved the problem for me.
but i was not able to use that particular fertilizer again, it was a fert/vitamin mineral additive especially for goldfish, but alas it just made all of my fish appear ill.