Because it is difficult to control for the average 50-120g hobbyist with minimal bio loads and a bunch of frags. But my main issue isn’t gfo, it’s the philosophy that po4 needs to be this or your sps will die. Drives me nuts. I was one of those hobbyists that listened to all of that advice from 2017 to 2018 that the harder I tried to chase n/p the more died. No more gfo and nopox= 95% success/thrive rate. So I’ll never use it again. I use skimmer, fuge and LC to keep po4 between 0.10-0.25Let me ask, why do you think gfo is bad