po4/phosphate- measurements of .02ish is optimum. However up to .08 is fine.
no3/nitrate- should be around 2ppm - 5ppm for optimal coral coloration. If 2ppm or below is observed for longer than a week, corals will pale
alk/alkalinity- between 7.2 and 7.8 at all times. At 8 or above sps get irritable. This could also be the issue with ultra-low nutrient systems and high alk where burnt tips occur due to the skeletal formation of calcium carbonate forming faster than the photosynthetic ability of the coral via the zooxanthellae