Are you growing turf grasses or macro algaes ? Most any photosynthesis will require some phosphate. Just considerably less than we are use to seeing in FW planted tanks. Zero phosphate on the other hand won't work ither, but it's pretty hard to acheive Zero in a closed system so it's a safe margine for most reef tanks. If on the other hand you are cultivating macro algaes, or turf grasses you're going to need more phosphates. Saltwater eco-systems are alot more specific when it comes to nutrient potential. This was a pitfall of early minireef sytems. Many people found out the hard way. HTH. Prof M