Anytime you’re over your target level.
GFO gets a bad rap on here cuz noobs don’t know how to use it; I run it 24/7 365. Nothing dies, but why?
Agree 100%. I run a gfo reactor 24/7 as well. I use 4 tablespoons in a 112 gallon system and change it every week or so. Many reefers misuse GFO, then blame it for coral loss. It's a powerful tool and works well in small quantities. My Cherry Bomb tripled in size in 4 months so GFO obviously didn't hurt it at all.