Unless you start to have real algae problems there's no reason to measure phosphate. It's kind of a pain and adds to the cost. Soft coral can handle it. You should measure your calcium and hardness levels. Many soft corals like colts, kenya trees, and others contain tiny calcium carbonate needles called sclerites. Without proper calcium and hardness levels they are unable to produce these sclerites and won't grow properly. Ideal levels are:
Ammonia: 0 ppm
Nitirite: 0 ppm
Nitrate: <20 ppm
Calcium: ~450 ppm
dKH: 8
Salinity: 34-36 ppt
Phosphate (if done): as close to 0 as possible
Many people try to maintain a salinity lower than 35 ppt, which is the salinity of natural seawater. This stresses the coral, and causes them to not thrive as well as they could.