GFO - coral reaction

I have been battling red slime in my reef aquarium for a couple of months and after my water change this weekend decided to reinstall my GFO reactor to combat phosphate. My phosphates test show 0 but I read that as long as i see red slime there has to be some in my tank. I used to use GFO before but due to hurricane Irma I lost all my fish and had to start over. I had not reinstalled the GFO because my soft corals were growing much better without it. I have mushrooms, hammer, frogspawn, candy cane and ricordea. I had forgotten how much to use and went to the BRS website which said 1 1/3 cup for my 75 gallon tank/10 gallon sump. Within 15 minutes all of my corals closed up. Looked like they were going to die. I shut off and within 24 hours they are back to normal. Reading various forums i get so many disparaging opinions as to the amount of GFO to use- now I am totally confused!
How much do you guys think I should start with? How fast should it be increase and to how much? Maybe I did not rinse well enough?