5.5 locks out a lot of stuff from what I've read, so I usually keep it around 5.7 or 5.8. Since my meter took a crap on me, I have to rely on a liquid tester so its harder to get it right at 5.7 or 5.8 so I usually just settle for around 6. I am going to add some of my veg nutes once the lights turn on and I can get in there.