Ok, so while talking to Rebecca, I realized that I need new bulbs for my MH lamps... The colors of my SPS were not so intense anymore and some where starting to turn brown.
So, I bought new bulbs and put them in. After a couple of days, they still didn't look too hot but I thought that that was still a result of the bulbs getting old. 2 days ago, I come home from work and most of my SPS were bleached. :(
I guess the new bulbs were too intense for the corals.
Lesson leared: Make sure you acclimate your corals to new lights/bulbs!