Yes — some said that fish(esp Yellow Tangs) may die from particulates getting in their gills. Seems like an urban legend to scare little children? Anyway, if used as directed, I’ve found it to be extremely effective and no mess! Start with a 1/4 dose and slowly ramp up as needed. After awhile, you know how many drops are needed for your tank. YMMV but I will never go back to GFO/refugium/carbon dosing if I just want to reduce PO4.This is great stuff!!!! But use caution.