Basically anything under .05 phosphates and between 3 to 10 nitrates. I don't chase the numbers, kind of let the corals tell me what they need. If they start to brown, I start nutrient export run my skimmer and refugium light 24/7 and do water changes. If they start to pale I start nutrient import, turn my skimmer and refugium light off. I guess you can say I'm a lazy reefer. I find when you keep it simple and let your tank do what it do, things work. When you start dosing and tweaking this and tweaking that, that's when your system levels goes out of balance and you start getting nusience algae and corals start to brown out and die.